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View Full Version : How to change gear ratio?


Gabe Athouse
07-03-2019, 04:24 PM
I wish to change my ratio, whatís the process with the ecu etc?

Midwestdrifter
07-03-2019, 06:54 PM
I have not heard of anyone successfully doing it on the T1N. I believe you need at least a matching TCM, possibly a cluster change.

As mentioned in the other thread, I have designed a canbus bridge which modifies the ESP canbus frames for the TCM, and should correct for a changed ratio. I have not done the final testing however, so it will be several months until I have transfer case, and do a final test.

If you change the ratio without the TCM knowing, it will cause trans LHM after about 15mph. The only fix is a TCM which is correct for that ratio. The TCM cannot be reflashed.

Gabe Athouse
07-03-2019, 06:58 PM
Dang okay thanks. Hurry up on that conversion kit!!

Midwestdrifter
07-03-2019, 07:06 PM
The code is written, I have tested the unit with a 1:1 ratio without issues. I just don't have a tcase installed in my van, so I can't see how it will behave. I am pretty confident it will work though, as a similar approach was used by Allrad for their T1N auto 4x4 conversions.

Gabe Athouse
07-03-2019, 07:23 PM
The code is written, I have tested the unit with a 1:1 ratio without issues. I just don't have a tcase installed in my van, so I can't see how it will behave. I am pretty confident it will work though, as a similar approach was used by Allrad for their T1N auto 4x4 conversions.

We (me) have been wanting a solution for this for years and years. I canít fathom owning a newer sprinter with all the silliness they have, nor can I afford one anyway. So, good work and keep it up! I will buy the first kit!

Gabe Athouse
07-03-2019, 07:33 PM
I’ll stick with 3.73 for the time being then. Thanks.

Midwestdrifter
07-03-2019, 07:45 PM
What ratio would you be considering? 4.11? I was looking at GM dually axles from the GMT900 trucks, they looked like they would fit with modified spring perches. Lots of gear ratio options for the sterling rear axles. Lockers too. You would need a tone ring converter box (which I am also developing).

Gabe Athouse
07-03-2019, 09:40 PM
I’d guess for most purposes the 4.11 would be best. I feel like I’m a tad over geared with my 235/85-16 on my sprinter with 3.73’s. So the 4.11’s would get me closer to stock but still slightly over geared for high speed economy. Folks who want to run bigger tires and tow heavy would likely want even lower gears.
So you need to replace rear axle too I take it? If that’s the case, and you’re lookin at dually, consider this: you know how the dually American trucks have those big shapely woman hips on them? And our sprinters dont? Well that’s because the sprinters push the dual wheel inward rather than outward, so I’d guess a gm axle would need to be shortened quite a bit to get inside the fenders. Not impossible but food for thought. You’ve probably already thought of that though.
Why does rear axle need replaced?

Another huge HUGE benefit of replacing axle would be the options of torsen LSD and lockers. For me, living on a hill with gravel and in a rainy climate, the RWD vehicles I’ve fitted with torsen rear diffs have been completely transformed and became vastly more competent in those environments. Imagine backing an empty pickup truck uphill on gravel. Almost impossible right? With trutrac installed it feels like you have AWD.
But, in my quest for a better off roading sprinter van, I’ve come to find ZERO options for any LSD or locker. Real bummer.

Midwestdrifter
07-03-2019, 09:54 PM
I don't need to replace the rear axle, I was suggesting that as an option if you wanted odd gears. The suburban 2500 rear axle is almost a perfect fit for the T1N 2500. 3500 sprinters would need a custom narrow spring/shock attachment, but you could run 315 tires for the same load with single wheels.

Since I am putting GM IFS on the front of my sprinter, I made a speed sensor converter box, so I don't have to replace the GM tone wheels (55 tooth).

I did consider a GM rear axle for slightly better brakes, and locker/LSD options. Maybe farther down the road if my MB one wears out.