Help with gear ratio when engine/transm. removed

L

Lee Mitch

Senior Member
I am trying to determine the differential gear ratio and need a little help. My engine and transmission are removed and I was wondering how I could calculate the ratio by watching the driveshaft turn in comparison to the rear tires. I was assuming that I could watch the shaft turn 2 complete revolutions and watch how much the rear tires rotate (like you do with the engine turning 2 complete revolutions) and I would have my answer. I was figuring that in that configuration I would be in 3rd/direct drive. Obviously I am incorrect. Any help out there? Thanks in advance
 
Lee
Almost got it
Jack up both rears off the ground and mark both wheels
Best to mark where you can see both together
Turn driveshaft and count revolutions
If 3.78 rear gears you will turn driveshaft 3 and 3/4 revolutions

The reason for marking both wheels is you can recover in case a brake drags and moves differential

HNY J
 
John,
Are you saying that 3.78 turns of the driveshaft should be 1 complete revolution of the tire/s? That does make sense.

So that would be equivalent to being in 3rd gear/direct drive I suppose.


Thanks
 
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Thanks JW. I examined that thread previously and couldn't find what I needed, specifically my engine and transmission being removed.
 
Lee Mitch said:
Thanks JW. I examined that thread previously and couldn't find what I needed, specifically my engine and transmission being removed.
Click to expand...

OK got it. Since the transmission is straight thru, one rotation of the crankshaft is the same as one rotation on the drive line.

If you have a 3.78 rear end. The rear wheel will rotate once for every 3 3/4 rotations of the drive shaft.
If you have a 4.11 rear end the rear wheel will rotate once for every 4 rotations of the drive shaft.

You have to have one rear wheel on the ground and one rotating while doing this test.
 
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Jack up only one rear wheel. It will turn 2 times for one turn of the ring gear. If you jack up both wheels one can, and likely will, turn more than the other and it will be difficult to keep track.
 
If you have a 3.78 rear end. The rear wheel will rotate once for every 3 3/4 rotations of the drive shaft.
If you have a 4.11 rear end the rear wheel will rotate once for every 4 rotations of the drive shaft.

You have to have one rear wheel on the ground and one rotating while doing this test.
Click to expand...
Actually with two rear wheels off ground you can (if the wheels turn together) see the 3.78 ratio.

If one wheel is held stationary, the ratio become 2 x 3.78. Or 7.56.

All part of the magic of the spider/differential gears. The spider carrier ALWAYS turns at 3.78. Hold the "circular rack" of one wheel gear stationary and you get 2x the motion of the other as it is pushed by the spider gears.


Joe K
 
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