Rear Gear ratio

[dbshelton]

I am swapping a 91.5 Cummins into a Bus. It weight about 13,500# loaded and now has a 4.63 rear gear which seems pretty steep for the diesel. Tires are 33" tall, OD is .71. What would be a good gear to install? I'm leaning towards a 3.73 or maybe a 4.10.

[dazedandconfused]

3.73 would give you better interstate speed and mileage. I don't know if there would be enough of a difference.

[dbshelton]

After doing some calculating, this is the engine speed in Overdrive:
Ratio 65MPH 70 MPH 75MPH
4.63 2212 rpm 2382 rpm 2552 rpm
4.10 1969 rpm 2109 rpm 2260 rpm
3.73 1782 rpm 1919 rpm 2056 rpm
3.54 1691 rpm 1821 rpm 1951 rpm

[dazedandconfused]

I'd go with 3.73 off that list. Is that including title height?

[ellis93]

I wouldn't do the 3:73,I'd run the 4:10 ratio to keep the rpms up. Carrying that much weight will cost you mileage and strain your engine topping hills and such with to high of a ratio gear.
With the truck in my sig and a 35' gooseneck I weigh in at 11.5k. At that weight I can top any hill running the same speed I started at the bottom with,and not get over 1100* on the pyro. That's mostly interstate driving running 70.

I'd almost be temped to run the low gears you have,just to see how she does

[dbshelton]

Yes, there's probably a good reason that Ford used the 4.63 rear ratio in both the 7.3 Powerstroke diesel and the 460 gas engine in the E-Super Duty chassis. It pulls and drives pretty good with the gas 460, I imagine it will be outstanding with the Cummins.