The Stovebolt Site Forums The Shop Area Driveline Differential swap on the 55 250

I read somewhere that I could swap out my stock differential on the GMC 250 (5:14) with a differential from a 67-72 3/4 ton automatic (4:10). This thing is like driving a tractor to town. I'd like to confirm this before I go out looking for one. Thanks...

Go ahead and look, but that particular third member is about as scarce as a chicken with lips! It was only used in certain 3/4 ton vehicles with an automatic transmission. The entire center section, not just the ring & pinion, will interchange. You'll be looking for an H0-72 rear end, I believe.

When I ran a dirt track racer in California, we scavenged up every 4.10 3/4 ton rear end we could find- - - -it worked very well on 1/4 mile tracks behind a Muncie 4 speed in 2nd. gear.
Jerry

Thanks Jerry.... I may have actually found one. Do I have any other options shy of changing the entire rear end? I've already upgraded my brakes and bearings and don't want to have to weld in braces for the leafs in a swap.

The center section is a direct swap- - - -no need to change the axle housing or brakes. The HO-52 and HO-72 center sections do not interchange parts, but the complete centerpiece is a bolt-in swap.
Jerry

Hey Dave, go to the tech tips at the top of the page, click on axles and you will find out how this is done. It's easier than you think.

If you do the swap, you will pick up about 20% top end speed if you do the math. You can add to this by increasing the diameter of your tires. When I changed out the 4.56 to a 4.10 on my '57 3600, I gained 10% and picked up another 10% by switching from 17.5" rims with bias ply tires to 19.5" rims with 8R19.5 radials. The truck runs very comfortably with this combination.

The differential swap is an easy Saturday morning project. Unless you have arms of steel, I recommend having a tranny jack with some wood scraps to use as shims to make the swap.

I had some trouble finding all the correct gaskets when I did the swap about three years ago. If you do a search on this site, I posted all the correct current gasket reference numbers to save someone else the trouble.

Good luck,

Matt

What rear is currently in the truck?
I don't know about your 55, but the later GMCs had Spicer rears which would not apply to the tech tip referenced, although a ring and pinion swap should be possible.

I'm not sure... but from the reading I've done it looks like it may be 5:14. I pulled the original split rims off of it... they sat at close to 34 inches tall, and put on a set of tires that were only 29 inches tall so I figured that made a pretty big difference. I replaced those with tires that were almost 32 inches tall and it did help but I'm still screaming at 45 MPH.

Check out the thread in the one ton section. Someone is wanting your 5.14 gears to swap with 4:10.

Check it out

Thanks....