Rear axle rate

It is for heavy haul. I have 3:55 with cummins and i really like it when i am east coast mountain. But i think 3:36 will be better for fuel also for running higher speed in the south or west states.

 

The Family 1 (450 hp and below) ISX engines are cruised ~1400 RPMs. Lot of fleet trucks have been running 3.25s and 3.36s with these.
The PACCAR MX is cruised even lower - I'm told 1100! The trucks Navajo ran with the Gen 1 PACCAR engines ran 3.55 - 3.36 rears. Now, Rocky Mountain doubles trucks run the MX13 with a 10 speed autoshift and 3.36s. Line haul trucks with a 9, 10, or 13 speed manual are running 3.08s, while the line haul trucks with the 10 speed autoshift are running 2.64s (with a .74 OD trans)!!!! I don't really speak to the ops side of that company, so I don't know what they're averaging for fuel mileage... even if I did, I couldn't tell you anyhow on account of the nondisclosure agreement I signed when I first started working there. Interestingly enough, the drivers running the Rocky Mountain doubles mostly seem to prefer the new RMD trucks over the old T800s which ran a 450 ISX, 10 speed manual, and 3.90 rears.