Engine RPM, Velocity, Axle Ratio

[jdcaples]

I've seen threads about rear axle ratio, speed and mileage.

I'm posting this because it was a question I had and didn't have an answer until received my van:

Make: Freightliner
Model: 2500 Cargo
Wheel Base: 144"
High Roof

Rear Axel: 3.92
Engine: v6 3.0 L Diesel

The speed comes from the instrument cluster - digital readout, not the analog dial.

RPM comes from the analog tachometer.

Speed: RPM

55 mph: 2000
60 mph: 2200
65 mph: 2400
70 mph: 2600 (I think)

My casual observation: seems to be a linear scale.

-Jon

[mattl]

Of course it's a linear scale :P

[Expert Marine]

I think we need 6th gear!

[Mrdi]

What seems to be be an optimum RPM for the V6 in terms of effeciency on a flat surface.
Going up a hill?
3.91 Rear end.
If the rear end ratio is NOT posted on the window sticker along w/ the other options, then it is a 3.91 by default?

[jdcaples]

The default rear axle ratio on 2007 NAFTA units was 4.18, erroneously published as 4.10. You had to order 3.92 (and pay $50.00) if you wanted a taller ratio.
Starting with model year 2008, the default was 3.92.

The hardware is labeled, on the passenger (NAFTA) side of the differential housing.

-Jon

PS: Personally, I think 1800 - 2200 RPMs is optimal.... maybe someone will disagree with me, but it seems like fuel economy starts to suffer above 2200... ok, maybe 2400 RPM... I haven't done a study and I suck at writing grant proposals.

[Diamondsea]

I have a scan gage and I also check fuel by the tank fill up. My feeling is also that 1800 to 2200 is the most fuel efficient. High speeds really hurts economy as the air resistance goes up by the square of the speed I believe but could be wrong. My experience and gut feeling is that going from 55 to 65 MPH reduces MPG by 10 percent and going from 65 to 75 reduces another 10 percent. I have done measurements on the flat long stretches of Rt 95 from Virginia south to Florida using a full tank and cruise control at each speed. My economy goes from 21 MPG down to 17. Dropping 10 MPH for 6 hours adds another hour to a short days drive! I am retired and in no hurry so I like to take it easy in the slow lane and let everyone pass. I do watch to see if I am holding up traffic and if so I will speed up or pull off to let the folks go by. It's very relaxing in the slow lane at 60 MPH! I have an 09 short, tall.