The Stovebolt Site Forums The Shop Area Engine Shop 1949 3/4 ton GMC- Which differential will swap

OK--I gave up after searching for quite some time. The info is probably somewhere on a post, but I could not find. I have a 1949 3/4 GMC truck with a 6.14 rear end and have found a higher gear ratio rear end in a 1970 GMC 3/4 ton with a positrac rear end. Will that differential swap into my 1949? Both back plates are 10 bolt, but the pinion shaft looks smaller on the 1970 model. Not all apart yet, so can not confirm. It would be good to know before I start taking all apart. I have seen posts that Chevy differntials will swap up to 1972, but not sure if all GMC models will work. I do not want to swap the whole axle, just the differential. Sorry about a possible duplicate post, but I would appreciate some advice.
Thanks

I have not heard that the Chevy and GMC differentials for 1970 were different. I also didn't think that they came with positraction. The full floating front loaders from 67 to 72 have pumpkins that will bolt in to the 49 3/4 Ton. They made 4.10 gears for the 3/4 Tons with automatics.

By the way the standard ratio for a 49 3/4 Ton would be 4.57 and the 5.14 is for the 1 Ton.

Most, if not all, 3/4 ton and 1 ton GMCs from the '60s up to the '70s used a Spicer/Dana axle instead of the Eaton rear that the Chevy used. Does the rear end that you have found have a drop out 3rd member? If it does, then it is an Eaton and should work. If not, then it is a Spicer/Dana (most likely a 70) and willnot, unless you swap the entire rear.
Side note, most Chevy trucks with the Camper Special package were also equipped with the Spicer 70. They basically used the GMC leaf spring chassis.

Gentlemen--Thanks for feedback on both. You are correct that the standard ratio for this truck was 4.57 or 5.14 per my manuals. My nephew and I checked the ratio and it came out over 6 so unsure if it was swapped out or if we just miss counted. The new rear end did come out of a 1970 GMC 3/4 ton(saw the title) and is definitely a positrac. It does not have a drop out 3rd member so must be the Spicer/Dana rear end. I have not been able to clean it up enough to see if it has any identifying markings, but sure that it will not work without swapping my entire rear end. I have new Huck brakes, drums turned, brake lines, etc, so just wanting to swap out the differential on current truck. Sooner or later something will come up, but many places scrapped out old stuff back when scrap metal prices were high. I have a couple of people looking so expect to find one soon. Thanks for the help and advice while learning about this old truck. Thanks
Willie

Good luck in your search. If you find more than one, Post it! There are a lot of us looking for the elusive 4.10 rear!