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Turbo John
02-01-2013, 09:25 PM
I am curious what kind of speedo/rpm your getting. Mine seems to be very inaccurate.

I am doing a tune for a certain rpm.

My rear end ratio per window ticker is 3.72

My speed shows the following

2150 shows 60mph

2300 shows 65 mph

2500 rpm shows 68 mph

I did not gps this.

tire size is stock or 29.28 dia

if you load this info into a gear calc......it shows 65 mph is around 2600 rpm

I am assuming my gear is correct for make and model

This is telling me my speedo is 8 mph off?

what have you guys found?

WPJ
02-01-2013, 09:47 PM
My Speedo consistently is showing 5km/h faster than the ecu is displaying via ultra gauge and my GPS. Dealer says they have +/-7km/h at 100km/h error is considered normal.

autostaretx
02-01-2013, 10:09 PM
I also monitor my speed with my ScanGauge... like you and WPJ, my dash speedo reads 5% high (so it says 63 mph when i'm really going 60).
My GPS and the ScanGauge and roadside "your speed is..." radar signs agree that the speedo is wrong. The odometer is accurate.
I also did an Excel spreadsheet for the gears (here: 50044 ), note that it's tailored for 15 inch wheels (hence the tires' revs per mile).
My tachometer (and scangauge RPM reading) both agree with the spreadsheet.
For us 15 inch wheelers, if we upgrade to 16 inch it tends to make the speedometer accurate, and then the odometer reads "low"

--dick
p.s. for what it's worth, my Kia's speedo reads high, too.. and, unlike the Sprinter, it's not a consistent 5% across the board... it reads about 10% high at 30 mph, and 5% high at 60 mph.

sailquik
02-01-2013, 10:39 PM
Turbo John;

"My rear end ratio per window ticker is 3.72"

The actual rear end ratio is based on
3.727:1 Ratio= Ring Gear Teeth =41/Pinion Teeth=11=3.727272727: 1 ratio

My speed shows the following

2150 shows 60mph
2300 shows 65 mph
2500 rpm shows 68 mph

My calcs (based on 3.717:1 rear end ratio and 29.28" tire Outside diameter) result in the following:
60 MPH= 2123 engine RPM
65 MPH= 2426 engine RPM
68 MPH= 2406 engine RPM
70 MPH= 2477 engine RPM

I did not gps this.
Might be a good idea to get a handheld GPS or take someone who has one for a ride to see if the speedometer is really that far off.

tire size is stock or 29.28 dia
How did you determine this diameter?
What is the OEM tire size and what brand/model tires are you running?
Roger

Turbo John
02-01-2013, 10:53 PM
Turbo John;

"My rear end ratio per window ticker is 3.72"

The actual rear end ratio is based on
3.727:1 Ratio= Ring Gear Teeth =41/Pinion Teeth=11=3.727272727: 1 ratio

My speed shows the following

2150 shows 60mph
2300 shows 65 mph
2500 rpm shows 68 mph

My calcs (based on 3.717:1 rear end ratio and 29.28" tire Outside diameter) result in the following:
60 MPH= 2123 engine RPM
65 MPH= 2426 engine RPM
68 MPH= 2406 engine RPM
70 MPH= 2477 engine RPM

I did not gps this.
Might be a good idea to get a handheld GPS or take someone who has one for a ride to see if the speedometer is really that far off.

tire size is stock or 29.28 dia
How did you determine this diameter?
What is the OEM tire size and what brand/model tires are you running?
Roger

225/75/R16 is shown as that dia 29.28

I went out with my GPS just now and here is what puzzles me?

65 MPH with a dia of 29 inches and a gear of 3.72 shows on a gear calc= 2800 rpm

My gps at 65 showed no more than 2500 rpm....making the tach off 300rpm

So it shows that I do not have a 3.72 gear which in my mind is near impossible

2500 rpm at a true 65 MPH and the stock dia which I have.......shows it to be around a low 3.2 gear

I cant believe the tach is off 300

The speedo shows 68-69 MPH at a true 65 which concurs with most everybody.

But the tach is puzzling me. I measured my tires and they are right at 28.750 to 29.00

6500= 2500 rpm Thats with the GPS I have no way of verifying my tach accuracy and no way of verifying my rear gear without jacking it up and turning the tires a revoloution. I have to believe I have a 3.72 but the tach does no show it??

Turbo John
02-01-2013, 10:55 PM
I am using stock tire 225/75r 16

I did not do the long math. I used the internet calculators. They must be wrong.

Turbo John
02-01-2013, 10:57 PM
My calcs (based on 3.717:1 rear end ratio and 29.28" tire Outside diameter) result in the following:
60 MPH= 2123 engine RPM
65 MPH= 2426 engine RPM
68 MPH= 2406 engine RPM
70 MPH= 2477 engine RPM

Look at your 65 mph and your 68 mph rpm's ? You show less rpm for a higher speed. and not much more for 70MPH...please explain?

Turbo John
02-01-2013, 11:09 PM
every gear ratio charts shows I would have a 3.35 gear if my tach was accurate. I run exactly 65 mph at 2500 rpm.

so if I have the 3.72 then my tach is off 300rpm

I went out on the highway and stared at the Tach and GPS until my eyes bled!

I would like it if one of you T1N guys would go run your sprinter at a GPS 65mph and tell me what your tach reads?
Thanks

sailquik
02-01-2013, 11:21 PM
Turbo John,
"Look at your 65 mph and your 68 mph rpm's ? You show less rpm for a higher speed. and not much more for 70MPH...please explain?"

I jumped the gun and put something out before I back checked it.......Sorry, I've edited that error out!
Can you PM me your VIN #?
I'll run it through the MB Startekinfo site and see what your Data Card has for your rear end ratio and send you
the data card if you like?
I'd also like to back check your tire diameter on Tire Rack.com so I'd also like the tire brand and model.
I've found their Tire Diameters work very accurately with the spreadsheets I'm working on.
Roger

autostaretx
02-02-2013, 04:47 AM
I cant believe the tach is off 300
Here is yet another case where a ScanGauge (or DashDaq or UltraGauge) would give you solid numbers as the ECU sends them (which knows when the engine makes a revolution, since it's squirting fuel to match it).
Far better than the "analog" tach and speedo (the circuitry can try to move the needle, but it can't see where the needle went).
ScanGauges (etc) pay for themselves in fuel savings (through better knowledge), in avoiding trips to a dealer for quick Check Engine Light analysis and oh, so many other handy readouts.

Since i have 15 inch wheels and the 3.727 rear end, i see very close to 2400 rpm at 60 mph (measured) and just shy of 2600 at 65 mph.
I got my "revs per mile" from the tire manufacturer's data sheet.
Remember that it's not "diameter" that determines revs per mile, it's the *radius* from the axle to the ground.. the tire is squished a bit by the weight of the sprinter.
(at 55 psi, your roughly 1500 pounds per wheel is going to create a footprint of 1500/55= 27 square inches. So it's not much of a squish)

--dick

owner
02-02-2013, 04:58 AM
Ok mine is on well worn 225/70R15 at about 40psi. At a GPS speed of 65mph, 104kph, im usually at 2650rpm. This is using OBD2 rpm and GPS on my phone. The speedo os reading 111kph at this point.

Note that the torque converter never locks fully, so revs can vary based on load mostly.

Turbo John
02-02-2013, 12:12 PM
Turbo John,
"Look at your 65 mph and your 68 mph rpm's ? You show less rpm for a higher speed. and not much more for 70MPH...please explain?"

I jumped the gun and put something out before I back checked it.......Sorry, I've edited that error out!
Can you PM me your VIN #?
I'll run it through the MB Startekinfo site and see what your Data Card has for your rear end ratio and send you
the data card if you like?
I'd also like to back check your tire diameter on Tire Rack.com so I'd also like the tire brand and model.
I've found their Tire Diameters work very accurately with the spreadsheets I'm working on.
Roger

I will get you the info thanks

Turbo John
02-02-2013, 12:14 PM
Here is yet another case where a ScanGauge (or DashDaq or UltraGauge) would give you solid numbers as the ECU sends them (which knows when the engine makes a revolution, since it's squirting fuel to match it).
Far better than the "analog" tach and speedo (the circuitry can try to move the needle, but it can't see where the needle went).
ScanGauges (etc) pay for themselves in fuel savings (through better knowledge), in avoiding trips to a dealer for quick Check Engine Light analysis and oh, so many other handy readouts.

Since i have 15 inch wheels and the 3.727 rear end, i see very close to 2400 rpm at 60 mph (measured) and just shy of 2600 at 65 mph.
I got my "revs per mile" from the tire manufacturer's data sheet.
Remember that it's not "diameter" that determines revs per mile, it's the *radius* from the axle to the ground.. the tire is squished a bit by the weight of the sprinter.
(at 55 psi, your roughly 1500 pounds per wheel is going to create a footprint of 1500/55= 27 square inches. So it's not much of a squish)

--dick

My tires are toyo brand.......I measured with a taped measure........there max 29 inches even though they are called out at 29.3 by size

Turbo John
02-02-2013, 12:16 PM
Ok mine is on well worn 225/70R15 at about 40psi. At a GPS speed of 65mph, 104kph, im usually at 2650rpm. This is using OBD2 rpm and GPS on my phone. The speedo os reading 111kph at this point.

Note that the torque converter never locks fully, so revs can vary based on load mostly.

I thought this trans locks up........especially in 5th at cruise?

lightwave
02-02-2013, 01:06 PM
I would check my DAD in the special functions>vehicle coding and enter the proper codes for tire size and rpm. I think this could cause a discrepancy


___________________________________________
2006 2500 118wb standard roof cargo 109,000 miles

Turbo John
02-02-2013, 04:26 PM
I would check my DAD in the special functions>vehicle coding and enter the proper codes for tire size and rpm. I think this could cause a discrepancy


___________________________________________
2006 2500 118wb standard roof cargo 109,000 miles


I have no DAD....there is something amiss
I have checked this rpm vs gps vs supposed rear gear several times.

What I do not know for sure is............rear gear and tach.

I do know tire size and MPH

I am going to rotate the rear end so I will know this for sure......then if the tach is off it is off.

I just do not know for sure if the convertor locks 100%.I feel that is does.

sailquik
02-02-2013, 05:05 PM
NAG-1 torque convertors do not lock up solid...the slip rate is normally ~ 5%, but heavy loads..head winds..etc., will cause them to slip a bit more.
Do a search on "Torque Convertor Clutch" to see Dr. A's very definitive research and conclusions on
Sprinter/NAG1 Torque Convertor Lock up (Dated 10-25-2009...10:01 PM).
If you had a ScanGauge II you would see your digital engine RPMs drifting up and down by around 10-20 RPM.
GPS speed can be rock solid to 0.1 mph but the revs still drift up and down by around 3-5 RPM above and below the
Cruise Control set point. It's just your engine management working all the time to give you as close to the set point
speed as possible.
Even more remarkable is to watch the % engine Load vary up and down in response to wind gusts,
minor gradient changes in the road.
It's interesting to watch the ECM keeping track of the % LOAD and varying the boost and
fueling rate to keep the speed precisely the same in synch with the varying external factors
(wind/grade/other variables) which neither the driver or engine management can control.

I suspect your tachometer is reading correctly.
I know your speedometer is off by some # of MPH....nearly all Sprinters read a few MPH higher than GPS/Actual speed.
MB decided this is a good way to prevent you from getting speeding tickets. If you think you are "pushing the limit" but
are actually going a few mph less....that sort of thinking.
Does the TOYO website give you revs/mile for the tires you have?
How will rotating your rear tires affect your MPH/RPM.....there's a differential between them, so the drive shaft revs will
stay the same regardless of which tire is on which side?
Rotate the fronts to the back and maybe a tiny impercetpable difference if the fronts are less/more worn.
Roger

Turbo John
02-02-2013, 05:24 PM
NAG-1 torque convertors do not lock up solid...the slip rate is normally < 5%, but heavy loads..head winds..etc., will cause them
to slip a bit more.
If you had a ScanGauge II you would see your digital engine RPMs drifting up and down by around 10-20 RPM.
GPS speed can be rock solid to 0.1 mph but the revs still drift up and down by around 3-5 RPM above and below the
Cruise Control set point. It's just your engine management working all the time to give you as close to the set point
speed as possible.
Even more remarkable is to watch the % engine Load vary up and down in response to wind gusts,
minor gradient changes in the road.
It's interesting to watch the ECM keeping track of the % LOAD and varying the boost and
fueling rate to keep the speed precisely the same in synch with the varying external factors
(wind/grade/other variables) which neither the driver or engine management can control.

I suspect your tachometer is reading correctly.
I know your speedometer is off by some # of MPH....nearly all Sprinters read a few MPH higher than GPS/Actual speed.
MB decided this is a good way to prevent you from getting speeding tickets. If you think you are "pushing the limit" but
are actually going a few mph less....that sort of thinking.
Does the TOYO website give you revs/mile for the tires you have?
How will rotating your rear tires affect your MPH/RPM.....there's a differential between them, so the drive shaft revs will
stay the same regardless of which tire is on which side?
Rotate the fronts to the back and maybe a tiny impercetpable difference if the fronts are less/more worn.
Roger

Well tire Dia is easy to measure. I raced cars for 35 years and we went by roll out rather than a measured dia. This is actually more exact. But I am off at least 300 rpm from where it should be. If the convertor is slipping it would even show higher rpm instead of a lower rpm. At a GPS of 65MPH my tach shows exactly 2500 rpm
A 3.72 gear would be 2800 rpm and if the convertor is slip sme 5% it would be even higher.
If I did not know better I would say I had a different rear gear.

sailquik
02-02-2013, 05:32 PM
John,
PM me your Vin # and we'll see what it came from the factory with!
AFAIK a 3.727:1 (3.72) ratio is as high a rear gear as you can get in
a NAFTA Sprinter.
I guess the tach could have an issue.
Know anyone with a ScanGauge II/Ultra Gauge/Dash DAQ or any generic scan tool.
You should be able to get the digital electronic RPMs for comparison.
Roger

sailquik
02-02-2013, 05:42 PM
John,
Along with your Vin# can you PM me a link to the Speed conversion calculator you are using?
All of my calculators (works in progress, but getting more accurate all the time) seem to indicate
that your RPMs @ 65 MPH should be ~2435 with the 3.916:1 ratio and ~2280-2300 with the 3.727:1
rear end. Revs would be 2600 if you had the 4.182 Rear ratio @ 65 MPH and I know this to be true as
that's what my 2012 turns @ that speed.
Where did you come up with the 2800 RPM=65 MPH?
Roger

Turbo John
02-03-2013, 12:39 AM
John,
Along with your Vin# can you PM me a link to the Speed conversion calculator you are using?
All of my calculators (works in progress, but getting more accurate all the time) seem to indicate
that your RPMs @ 65 MPH should be ~2435 with the 3.916:1 ratio and ~2280-2300 with the 3.727:1
rear end. Revs would be 2600 if you had the 4.182 Rear ratio @ 65 MPH and I know this to be true as
that's what my 2012 turns @ that speed.
Where did you come up with the 2800 RPM=65 MPH?
Roger

Here is the gear calc

http://www.richmondgear.com/101032.html

Turbo John
02-03-2013, 12:56 AM
John,
Along with your Vin# can you PM me a link to the Speed conversion calculator you are using?
All of my calculators (works in progress, but getting more accurate all the time) seem to indicate
that your RPMs @ 65 MPH should be ~2435 with the 3.916:1 ratio and ~2280-2300 with the 3.727:1
rear end. Revs would be 2600 if you had the 4.182 Rear ratio @ 65 MPH and I know this to be true as
that's what my 2012 turns @ that speed.
Where did you come up with the 2800 RPM=65 MPH?
Roger

here is the vin number

WD2-PD744845-662036

Turbo John
02-03-2013, 12:58 AM
FOR THE GEAR CALC

I used tire dia 29
MPH Gps 65
Rear gear 3.72

This would be for a manual trans.....which means a locked up 100% convertor and it shows 2802 RPM

Nobody else agrees with this rpm for this gear/tire /speed

autostaretx
02-03-2013, 01:52 AM
We know the size ( 225/75/R16 ) .. but for those of us who haven't stared at the tires... what make/model are the tires?
(so we can look up the "turns per mile" in the tire databases)

--dick

sailquik
02-03-2013, 02:48 AM
Dick,
He has Toyo 225/75R16 tires, but I don't know the tire model.
Turbo John,
Your Sprinter came from the factory with a 3.727:1 Rear differential ratio....
per your data card.
PM (up at the top of the page) me your email address and I will return your entire
MB build "Data Card" in .pdf format.
I'll have a look at your formula, but somehow it isn't calculating correctly.
I think your tachometer is providing you the correct RPMS for 65 mph, but your calculated RPMs are
off by quite a bit.
Roger

sailquik
02-03-2013, 03:01 AM
Turbo John,
Your tachometer is right on!
The problem with your calculations is that you left the final drive ratio (no overdrive) @ 1:1.
Your Sprinter NAG-1 transmission is a 0.83:1 OVERDRIVE ratio in top gear (5th or Drive).
So, your calcs with 1:1 final = 2802 RPMs.
If you change your calculation to account for the overdrive ratio (0.83:1) = 2325 RPM @ 65 MPH which is I suspect about what your tach is reading....right?
So, you actually calculated the RPMs for 4th gear which is 1:1 in your NAG-1.
Others take note as this is good info if you wish to manually downshift your Sprinter.
5th gear downshift to 4th gear will increase your RPMs from 2325 to 2803= + 478 RPM!
Roger

autostaretx
02-03-2013, 04:21 AM
Without the model, it's hard to guess which Toyo tire he has (the Open Country A/T II?)
If it is an OC A/T II, then it's rated at 709 turns per mile, per http://toyotires.com/tire/pattern/open-country-at-ii-on-off-road-all-terrain-tires

Feeding the 709 into the spreadsheet i posted above, we get: (5th gear is just above the blue lines)

50076

TJ said: " I run exactly 65 mph at 2500 rpm."
*but* he also said (in the first posting):
--------------
My speed shows the following
2150 shows 60mph
2300 shows 65 mph
2500 rpm shows 68 mph
-----------------
... and that last line matches the "0.83 gearing at 2500 rpm gives 68.4 mph" near the bottom center of my chart (boxed in red).
(my chart's top-half says that he should be seeing 2200 rpm (2193.4) at 60 mph.
..which agrees with sailquik, too.

In 4th gear (boxed in green), he'd be going 68 mph at 3000 rpm.

My foggy memory seems to remember Andy once writing that the NAG1 tranny (or torque converter) slips up to 5% when "locked".. so that's a possible error of 125 rpm right there (at 2500 rpm).
--dick

Turbo John
02-03-2013, 10:17 AM
Turbo John,
Your tachometer is right on!
The problem with your calculations is that you left the final drive ratio (no overdrive) @ 1:1.
Your Sprinter NAG-1 transmission is a 0.83:1 OVERDRIVE ratio in top gear (5th or Drive).
So, your calcs with 1:1 final = 2802 RPMs.
If you change your calculation to account for the overdrive ratio (0.83:1) = 2325 RPM @ 65 MPH which is I suspect about what your tach is reading....right?
So, you actually calculated the RPMs for 4th gear which is 1:1 in your NAG-1.
Others take note as this is good info if you wish to manually downshift your Sprinter.
5th gear downshift to 4th gear will increase your RPMs from 2325 to 2803= + 478 RPM!
Roger


I have to laugh.......showing human imperfection. Me of all people should have known this. I have a 480LE trans in my Hot rod with an overdrive and have figured out gearing many times. duh........Thanks for the reminder about how we can forget such a large technical thing..........!
Thanks so much for clearing my head!

Turbo John
02-03-2013, 10:22 AM
Without the model, it's hard to guess which Toyo tire he has (the Open Country A/T II?)
If it is an OC A/T II, then it's rated at 709 turns per mile, per http://toyotires.com/tire/pattern/open-country-at-ii-on-off-road-all-terrain-tires

Feeding the 709 into the spreadsheet i posted above, we get: (5th gear is just above the blue lines)

50076

TJ said: " I run exactly 65 mph at 2500 rpm."
*but* he also said (in the first posting):
--------------
My speed shows the following
2150 shows 60mph
2300 shows 65 mph
2500 rpm shows 68 mph
-----------------
... and that last line matches the "0.83 gearing at 2500 rpm gives 68.4 mph" near the bottom center of my chart (boxed in red).
(my chart's top-half says that he should be seeing 2200 rpm (2193.4) at 60 mph.
..which agrees with sailquik, too.

In 4th gear (boxed in green), he'd be going 68 mph at 3000 rpm.

My foggy memory seems to remember Andy once writing that the NAG1 tranny (or torque converter) slips up to 5% when "locked".. so that's a possible error of 125 rpm right there (at 2500 rpm).
--dick

the first check I did not understand how far off the MPH was from Mercedes.....keeping me from getting tickets......thanks Mercedes!
This is dangerous information.......LOL

All makes sense now..mine seems to run about 3-4 mph high which concurs with normal speedos for these vans. I do believe the lock up converter in not locking up because I am seeing 2500 rpm at a gps of 65.

Overlooking the OD ratio was an oversight I have been around non overdrive transmission most of my life hence the brain fade........
Thanks Guys!

sailquik
02-03-2013, 12:45 PM
Turbo John;
Have a look at this posting please:

http://sprinter-source.com/forum/showthread.php?p=67705&highlight=Torque+Convertor+Clutch#post67705

Your torque convertor will never completely "lock up" because TC lockup is not a feature found in the NAG-1 trans/torque convertor!
Doktor A (Andy Bittenbinder) has thoroughly researched this issue and the above post clears up all the mis-information
that's floating around.
Roger

Aqua Puttana
02-17-2013, 07:40 PM
Per my datacard my 2004 has

AC6 AXLE RATIO I = 3.727

The other day I noticed that at 80 mph by the speedometer (77 mph by the GPS - I run basic LT225-75R-16 tires.) I was turning 3000 RPM by the dash tachometer.

That may be an easy to remember reference number set for some to use to help to determine if they have the 3.727 rear ratio.

If you can hit 3000 rpm and don't see 80 mph... or flashing lights of a police car behind you, then you have a another ratio. (3.916 or 4.182)

FWIW. vic

talkinghorse43
02-18-2013, 03:29 PM
Was it the same pre-DARF? Also, might depend somewhat on which tires you're running. I have Michelin A/S on one axle and Michelin A/T2 on the other (both same "size"). Depending on which is on the rear, rpm for mine will be 2550 (A/S) or 2490 (A/T2) at 70 indicated (67 GPS). My speedo looks for and tracks the fastest rotating tire or axle.

Aqua Puttana
02-18-2013, 09:33 PM
Was it the same pre-DARF? Also, might depend somewhat on which tires you're running. I have Michelin A/S on one axle and Michelin A/T2 on the other (both same "size"). Depending on which is on the rear, rpm for mine will be 2550 (A/S) or 2490 (A/T2) at 70 indicated (67 GPS).
I have no idea? :idunno: I doubt the DARF affects the "lockup" significantly at speed or there would be significant premature transmission wear.

Both the speedometer and the tachometer scaling are a bit rough. The general 3000 rpm/80 mph ratio should be hold true for the 3.727 ratio for the tire size. A different axle ratio would change those numbers quite noticeably. vic

My speedo looks for and tracks the fastest rotating tire or axle.
That's not what I recalled, but my information is not official MB so may be suspect.

50497

50498

vic

talkinghorse43
02-19-2013, 02:07 PM
I have no idea? :idunno: I doubt the DARF affects the "lockup" significantly at speed or there would be significant premature transmission wear.

You could get a rough idea by calculation. Using A/T2 tire specs for example, I calculate that the tranny input should be running 2450 rpm at 67 mph (GPS) and, for mine, the Scangauge reports 2490 rpm at the engine.

Both the speedometer and the tachometer scaling are a bit rough. The general 3000 rpm/80 mph ratio should be hold true for the 3.727 ratio for the tire size. A different axle ratio would change those numbers quite noticeably.

Don't know which tires you're running, but I did some calculating using Michelin A/T2 specs (710 revs/mile) and tranny input (from torque converter) should be 2820 rpm @ 77 mph w/3.727 and 2960 w/3.916. Certainly a difference the Scangauge should be able to show, but then there's the speedometer accuracy/precision to contend with; and the same for the tach if you're using that.


That's not what I recalled, but my information is not official MB so may be suspect.

What you have shown in the attachment is my experience; that the speedo tracks the fastest rotating wheel if all are not rotating at the same rate.

Aqua Puttana
02-19-2013, 08:21 PM
Well...

I know that the relative ratio of the transmission top gear, rear end, and tone wheel teeth on the wheel sensors doesn't change. I'm so used to the speedometer signal being related to the drive wheels that I figured the rpm to speedo relationship would remain constant.

I failed to consider a couple things...

The engine RPM change for a given speed is not really that big between the 3.727 and 3.916
ratio.

The speedo signal (under normal conditions) indexing from the front wheels would be affected by the tire size.

So I guess there is no reason for anyone to use my vehicle specific 3000 RPM/80 mph constant.

Carry on. vic

You could get a rough idea by calculation. Using A/T2 tire specs for example, I calculate that the tranny input should be running 2450 rpm at 67 mph (GPS) and, for mine, the Scangauge reports 2490 rpm at the engine.



Don't know which tires you're running, but I did some calculating using Michelin A/T2 specs (710 revs/mile) and tranny input (from torque converter) should be 2820 rpm @ 77 mph w/3.727 and 2960 w/3.916. Certainly a difference the Scangauge should be able to show, but then there's the speedometer accuracy/precision to contend with; and the same for the tach if you're using that.




What you have shown in the attachment is my experience; that the speedo tracks the fastest rotating wheel if all are not rotating at the same rate.

talkinghorse43
02-20-2013, 01:43 PM
The speedo signal (under normal conditions) indexing from the front wheels would be affected by the tire size.

It's more complicated than I ever imagined. In addition to the speedo following the fastest rotating wheel (could be front or back), the odo only looks at the front axle on mine. With the A/Ss on the front, the odo is dead-nuts. With the A/T2s on the front, the odo is slow (0.985 miles recorded/mile driven).

Aqua Puttana
02-20-2013, 04:18 PM
It's more complicated than I ever imagined. In addition to the speedo following the fastest rotating wheel (could be front or back), the odo only looks at the front axle on mine. With the A/Ss on the front, the odo is dead-nuts. With the A/T2s on the front, the odo is slow (0.985 miles recorded/mile driven).

Interesting. The book info I posted earlier in this thread does say that the front wheels are the preferred selection unless there are significant variations. "Significant variation" is somewhat objective. vic

talkinghorse43
02-20-2013, 04:31 PM
Based on my data, the significant variation threshold is somewhere <1.5%. And, what I remember of others putting different "size" tires on axles, there's also a variation threshold somewhere >1.5% where cruise, ABS, etc. stop working.

Turbo John
02-21-2013, 12:07 AM
Doctor A was spot on with a 5% slippage. I have run the math and thats about the TC slippage. I am very surprised they dont lock the converter up.

The only thing is the speedo is off. I drove past a speed detector today...........my 50 MPH is really 45 and my 68-69 is really 65 MPH

Too bad the speedo cannot be corrected........now I know why I never got a ticket in this van......Maybe its a good problem.......or no problem:clapping:

shortshort
02-22-2013, 01:00 AM
Tire slippage is about 10% on a motorcycle at warp speed. I wonder what it is on a sprinter going down the freeway.

talkinghorse43
03-11-2013, 09:45 PM
Running down the highway today I noticed the engine rpms are 2835 at 80 indicated. Signifficantly lessthan 3000.

sailquik
03-12-2013, 12:06 AM
TH43,
If you have the 3.727 gear set in your Sprinters rear end, that's exactly the correct RPM for 80 mph with 29.40" dia. tires (215/85R 16).
RPM 1st 2nd 3rd 4th 5th
2800 18.3 30.0 46.6 65.7 79.2
2900 19.0 31.1 48.3 68.1 82.0

Aqua Puttana
03-12-2013, 12:09 AM
Running down the highway today I noticed the engine rpms are 2835 at 80 indicated. Signifficantly lessthan 3000.
In that case you'd better get your Sprinter checked out by a dealership.

Only kidding. :laughing:


The speedo signal (under normal conditions) indexing from the front wheels would be affected by the tire size.

So I guess there is no reason for anyone to use my vehicle specific 3000 RPM/80 mph constant.

Carry on. vic
I presume that is using your Scangauge? I know that I can't read my my dash analog tach to XX35 on a good day. Thanks for the input. :thumbup:

vic

Adam12
03-13-2013, 12:43 PM
I was running 225/70/15 tires. My Speedo was 5 mph off. Top speed was 85 according to my speedometer with brand new tires In actuality it was 80mph..(according to multiple GPS units and radar speed displays near highway construction sites) .As my tires wore out just before replacing it went down to a 4 mph discrepency...64 was actually 60..Top speed on my speedo was displaying 84 and GPS and Radar was 80......
Recently I replaced my rims and tires with 16 inch MB rims and 225/75/16 tires.With Brand new tires - properly inflated- NOW when i pass radar speed displays and my 2 GPS displays it is Right on. 65 is 65. TOP SPEED AT 80 IS 80.
Will be curious to find out when my tires start to wear out if I start to get a 1 MPH change..:thinking:..

Aqua Puttana
03-13-2013, 01:10 PM
... Recently I replaced my rims and tires with 16 inch MB rims and 225/75/16 tires.With Brand new tires - properly inflated- NOW when i pass radar speed displays and my 2 GPS displays it is Right on. 65 is 65. TOP SPEED AT 80 IS 80.
...
I find it interesting that MB has designed in what seems to be a pretty consistent 3 mph + reading on the speedo regardless of speed range, but the odometer is accurate.

Next time you are on the interstate with nothing to do check your odometer against a 5 mile or so marker post run. I think you'll find that your odometer is now off a bit with the different wheels/tires. vic

Adam12
03-14-2013, 02:36 PM
Next time you are on the interstate with nothing to do check your odometer against a 5 mile or so marker post run. I think you'll find that your odometer is now off a bit with the different wheels/tires. vic[/QUOTE]

Vic . I never checked my odometer. I do a considerable amount of highway driving.Living near Rts 84,81,380 and 80. I will give that a check...!!!! THANKS
I find it very interesting that the speedometer was off that much with the 15" original rims and now switching to 16" rims its corrected. I always use cruise control and vary my speed up and down using the cruise adjustment to drive. I stop at the gas station every 3 days. I rack up the miles.Had my sprinter almost 2 years and have saved almost 15,000.00 in fuel costs vs my express van.. The Sprinter will pay for itself in the next year..That is my salvation..:clapping:
Time to hit the road again. late start today.. UPDATE :
Did a Odometer Check .. Ran 8 miles using mile markers. Went from mile markers 8.8 to 16.8 .My odometer showed 7.4 miles. I never checked my odometer while running on the 15 inch Rims and tires...... Just my 2 cents adjusted for inflation now valued at 2 dollars..

talkinghorse43
03-14-2013, 03:35 PM
TH43,
If you have the 3.727 gear set in your Sprinters rear end, that's exactly the correct RPM for 80 mph with 29.40" dia. tires (215/85R 16).
RPM 1st 2nd 3rd 4th 5th
2800 18.3 30.0 46.6 65.7 79.2
2900 19.0 31.1 48.3 68.1 82.0

EXCEPT:

- not really 80 mph, it's 80 mph indicated (speedo)
- rear tires are Michelin A/T2 225/75/r16 w/ 24/64" tread remaining (new, listed as 710 revs/mi, 29.3" dia, 16 or 17/32" (ave is 33/64") tread depth)
- 29.3" dia calculates to 688 revs/mi vs listed 710 - which is correct?
- present dia is 29.3" - 0.281" = 29" (calculates to 695 revs/mi)
- how about the currently-accepted view that the TCC doesn't ever lock (~5% slip at least)?

autostaretx
03-14-2013, 05:34 PM
- 29.3" dia calculates to 688 revs/mi vs listed 710 - which is correct?
- present dia is 29.3" - 0.281" = 29" (calculates to 695 revs/mi)
Neither?

Measure your center-of-axle -to- ground distance (i.e. the radius to the road)
That will be a more accurate measurement of how the vehicle actually rolls.

Even that is still not completely accurate, since the tire deforms (a leading bulging "bow wave" of tread) as it rotates.

You could also put a chalk mark on the tire (straight down), and then move the Sprinter straight forward a known integral number of turns (10?).
Then measure the distance along the ground traveled by those ten turns.
That will be the accurate (at low speeds... i have no idea how much "lift" you'd get by centrifugal force "pushing out" the tread belt) value for your tires on your sprinter.

--dick