The Stovebolt Site Forums The Shop Area Driveline Determining Ratio

I'm working on getting a '51 3100 back on the road and currently going through a 216/235 swap. I'm an amateur at auto mechanics so I'm relying on the excellent advice I find on this forum. I just removed the transmission and have a question. I pretty certain the transmission had a rebuild prior to my purchase of the truck and I'm not sure whether any other work was done on the driveline. I don't know if anything has been done with the differential. May be a dumb question, but is there any way to determine the gear ratio by rotating the driveshaft X number of turns, then measuring the rotation degrees of a rear wheel?

Yes. Easiest is to jack up one rear wheel while leaving the other on the ground. Mark the jacked up wheel and the transmission end of the driveshaft so you can count rotations from under the truck. Have your helper slowly turn the jacked up wheel exactly one turn while you count revolutions of the driveshaft. Divide the number of driveshaft revolutions by 2. The result will be your ratio. Most likely a little over 4 because the original rear end on '51 half ton trucks had a 4.11:1 ratio.

Excellent. I didn't have a helper, so i put a chalk mark on the driver-side rear wheel and turned the driveshaft while watching for one rotation of the wheel. I only turned the driveshaft 2 turns plus maybe 15 degrees to get one rotation on the rear wheel. Both wheels are off the ground and only the driver-side wheel is turning. would it matter if, like you suggested, the wheel was turned instead?

Sorry, I did the math wrong. Should have been multiply by two instead of divide. Are you sure the passenger's side didn't move?

Chock one rear wheel, or stack up some boards under the tire so there's a little weight bearing on it. That will assure there's no movement as you watch the other wheel turn. A 4.10 rear end gear, which your rig probably has, will take slightly more than two turns of the driveshaft to make a single rear wheel do a full turn. The spider gears double the speed of the rear wheel if only one is turning.

That's something to remember with a vehicle that's stuck in mud or snow- - - -if a single wheel is spinning, it's moving twice the speed that the speedometer is indicating.
Jerry

I made a short video Video LINK showing what others have described above. The reason I turn the wheel 10 revolutions is because 3.42, 3.73, 3.90 are similar enough that you might get it wrong with just 1 turn. With 10 turns your math accuracy improves significantly.

Thanks for the great info all. The truck is currently up on jack stands with the wheels off but I will put a strap on or block up the passenger wheel to make sure it doesn't move. I'll go for the ten turns tomorrow and will see what we come up with.