The Stovebolt Site Forums The Shop Area Driveline 1940 GMC rearend question

question - i have a 1940 GMC 1 1/2 ton dully pickup and i'm wanting change out rearend to a later model 3.54 range and keep original 5 lug rims.anyone have an ideal of what year model or make to look for. or possibly use an adaptor to keep original rims.

Dumb question here... are you running the 228 or another gmc 6 like the 270 or 302? I don't think a 228 would pull the 3.54 gears well in a 1.5 ton truck.

Those 5 lug rims are about 20 inch diameter with about 35 inch diameter wheels.

Using an online calculator with 3.54 gear, a direct drive transmission, and 35 inch wheels... 65 mph looks like about 2200 rpms. A 228 would not pull that, but a 270 or 302 wouldn't have much trouble pulling those gears.

I have a 1940 3/4 ton which I have mocked up a NV4500 transmission swap. IMHO, A NV4500 would be a much better swap than an axle swap... unless you are swapping the axle because it is worn out. I chose 4.11 gears to go with my NV4500 and 302 gmc motor and 30 inch tall tires. I should be able to do 70 mph at 2350 rpm

I just read another thread that states you have a cummins and a 5 speed(NV4500?) 65mph looks like about 1612 rpm. The diesel should be able to pull that on the flat.

I don't have much experience with larger rear ends, but I remeber the 1 ton dodge diesel I got my NV4500 out of had an 8 lug 3.55 dually rear. Perhaps you can make an 8 to 5 adapter.

The 1 ton dully truck i removed the motor from had a 3.54 ratio rearend and at 65 MPH motor rpms were a little high.truck had no problem pulling what ever was put behind it.so i figured 20"rims with 34"dia. tires i would be able to trailer my 42 behind with ease. the 40 has 2 speed axel thats making a lot of noise and is geared to climb mountians.

If its a 40 model with the original engine, the only way it will pull a trailer with 3.54 gears and 20" wheels is down hill with a tail wind.

with cummins power- pulls up hill and down are no problem.