The MGA With An Attitude
Determining the FINAL DRIVE RATIO - RA-105

At 05:24 PM 8/17/04 -0500, John Barber wrote:
>"I was trying to find the discussions about how to run the test to find out what rear end ratio I have. Could someone run through that for me again?"

Lift one side of car and leave one rear wheel on the ground, or block one rear wheel so it will not turn. Mark the free rear wheel for orientation, perhaps rotate it to have the valve stem at the bottom. Mark the propshaft so you can count rotations, maybe a tape or chalk mark on the bottom.

When you rotate the free wheel, the ring gear will rotate at half speed because of action of the differential gears. So rotate the wheel two turns while counting the rotations of the propshaft. For a 4.300:1 ring and pinion ratio (43/10) the propshaft will rotate 4.3 turns.

When in doubt, if you want to be very accurate rotate the wheel 20 turns, count the rotations of the propshaft, and divide by 10.
For the 4.300:1 ring and pinion ratio the propshaft will rotate 43 turns.
For the 3.909:1 ring and pinion ratio (43/11) the propshaft will rotate 39.1 turns.
- - Etc. - -

If your car has a clutch type positraction or locked differential, lift both rear wheels off the floor so they can turn together. Also rotate the wheel(s) 1 turn or 10 turns vs the 2 or 20 noted above.

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