The Stovebolt Site Forums The Shop Area Engine Shop GMC 1ton rear

My friend is restoring his GMC 1ton 1961.
He likes to find out what rearend gear options he would have for upgrading to higher ratios.
He has 5.14 stock rear right now, but likes to be able to go on the freeways.

Any 3/4 ton differential gear set from 1946-1972 will interchange with your friend's 5.14:1 without too much dinking around.

Most 3/4 tons came with 4.55:1 but a few came with 4.10:1 or 3.90:1. (mostly the automatics)

Just keep in mind that that truck is geared like it is to get your one ton of cargo moving. If he's planning on using the truck as a truck, he'd probably be better off with the 5.14 gears. (He could probably get away with the 4.55 ratio, too.)

Also remember if you make it go faster than the brakes are designed to stop, you could be in trouble.

What Brendan said is true.

If you want to load it like a 1 ton, then when changing to a smaller number rear (like a 4.10) you won't be able to get your load moving (as well).

The 4.10 is available from people who part out(I got one recently) as is the 4.56. I've never seen a 3.90 available. I've heard of a 3.38 but think it is a myth. Havn't seen one myself... only read about people who's second cousin talked to a guy who saw one at a swap meet 10 years ago.

Consider looking on Pirate 4x4 or other such boards. There might be someone who would LOVE to swap you a 4.10 for your 5.14. The 5.14 is RARE, and wheelers and boggers love that tough old Eaton axle and the 5.14 is the holy grail of gears for that axle.

If you just want to go faster, and still want to load it up like a 1 ton, consider buying one of those Spicer auxilliary transmissions that are in the for sale board... or some other aftermarket auxilliary tranny.

I have 19.5" tires. That helps things go faster.

What motor is he running?

I have a 1-ton that has 3.73 gears in the rear (swapped axle). The guy I bought it from said it was no longer drivable able he swapped rears.

If he is running a stock 235 he may want to think about that.

Can I throw a wrench in the works? Being a GMC, there's a good chance it may have a Spicer axle rather than the Eaton. If it is, most likely it would be a Spicer 60. Gears are readily available for those axles, check any rear end supplier or off-road shop.

The rear according to the manual is a Corporation HO72 and not an Eaton or Spicer.

Are there gears readily available anywhere or are there any specialty vendors?

The HO72 was manufactured for GM by Eaton (Detroit Gear and Axle). To my knowlege, parts selection is kind of slim for these rears, along with it's HO56 and HO33 little brothers.
I only threw out the Spicer suggestion as a possibility, as it isn't unusual to see them in '55-'77 GMC trucks.

I'd forgotten, but my '55 1st GMC had a Spicer under it when I bought it. I replaced it because the pinion bearing failed.