The Stovebolt Site Forums The Shop Area Engine Shop Rear End Gear Ration Identification

I have a 1950 3/4 ton and want to know what ratio the rear end has. It doesn't run right now so I can't tell you how fast it will go. My father in-law said when he had it, it would only go about 35mph. Anyone know a way to identify it without disassembly?

Also, I just found a replacement 4.11 pumpkin that came out of a 72 Chevy 3/4 ton Camper Special with coil spring rear end and automatic. Is this the correct replacement for my 1950?

The 3/4 ton rears typically had a 4.56 ratio in them, as stock/standard from the factory. The 4.11 wont gain you all that much more in the speed department, and there may be issues with '70's vintage carrier (open driveline) being fitted into a 1950, that, if original, should have an enclosed driveline. You'll likely need to swap a newer tranny in as well. HTH

AFAIK, 3/4 ton and up are open driveline starting in '50 or so..........it's the 1/2 tonners that have enclosed up to '54-'55 1st (some '55 1st are open).
Rear gears should be 4.57's, but may also be the dreaded 5.14's .
The newer third member should bolt-up; the u-joint size may be different, I am not sure (I may be thinking _Ford here).
To make sure about the ratio currently installed, try this:
"Jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full
revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 & 3/4 turns, you probably have a 3.73 gear ratio. Turning the tire for twice the
number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading."
(quoted from West Coast Differentials)

The 4:11 differential will bolt directly in. The truck it came out of strongly points to a Detroit Locker (No-Spin ) unit

If you have an open rear driveline (it should be) the u-joint size should be the same. For gaskets tell the parts store it's for a '70 3/4 ton 10 bolt full floater differential in a 2 wheel drive. The swap isn't that hard, I believe there is a link to Kook's website in the tech tip section.

Last night I checked the ratio per the instructions from gmc248. I got just a little over 4 revolutions out of the drive shaft for every one of the wheel. This would indicate to me that the rear is probably a 4.11 already. Wouldn't you agree?

Are you sure about the 4.11's? You had one tire on the ground? (if you have an open diff.)

Accuracy counts here to tell the difference between a 4.57 and a 4.10 (not 4.11). With two turns, the difference would be greater. That is 8.2 turns versus 9.14 turns.

I had to have one wheel on the ground and the other off to do this check. When I did, I got a little over 4 turns out of the drive shaft. I went ahead and turned the wheel twice and divided the rotations of the shaft by 2. It was almost exactly 4.

It sounds like you are a lucky man. If you take the back cover off, the large gear should have 41 teeth. That will tell you for sure. You probably couldn't count them just by looking. You would have to put a paint mark on one tooth and then turn and count.

You can count the teeth or you can look where the threaded end of the ring gear bolts come through the ring gear itself (teeth side). Look between the ends of the bolts and you will find the magic numbers you need to positively identify the animal you are working on
(all the methods suggested above can work to a point but there is nothing like numbers )

With the open diff., you have to turn the wheel (2) times and count the # of revolutions of the driveshaft. If you turn the wheel (4) times, count the # of revolutions of the driveline; then divide that number by two . This will tell you if you have 4.10 or 3.90 for sure.
If it's a 4.10, you would turn the wheel two times and the driveshaft would go around 4 times + a little more...........there is no 4.00 for GM.
There was a 3.90 (rare or custom made only). That's the one I would want.