spec'ing a shed hauler - axle ratio etc

[Btech]

Just gonna through my 2 cents in here, I do work for a guy that has a site prep business for storage sheds and he has 3 Peterbilt 348 dump trucks and they always pull a trailer with a skid loader on it.
All have the cummins L9 paired with a 6 speed Allison, not sure what ratio rears they have as I've never had a reason to check, but I would say with a 0.65:1 top overdrive ratio 3.90 would probably be ideal (1500RPM with 4.11 is 68MPH; with 3.90 is 72MPH; and with 3.70 is 76MPH)
Other than having to do manifold gaskets and the coolant line that goes to the compressor leaking that the L9 is notorious for, they have been great trucks for him.
And from doing road tests, the engine brake on them is no joke. it will almost throw you out of the seat when it kicks in if you don't realize its on. Only time we've had to do brakes on the trucks is because of age and cracking, they haven't come close to wearing the lining out.