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To positively determine your rear end ratio without tearing it apart and counting teeth, here's a method that costs nothing but 10 or 15 minutes of your time.
- Assuming you have an open (non-limited slip) differential, jack one rear wheel off the ground, blocking/chocking the other wheels so the car cannot roll and hurt you! Put the car in neutral and release the parking brake.
- Put a pair of alignment marks on the (rotating) driveshaft pinion dust shield and (stationary) differential housing. Without moving anything, put a similar pair of alignment marks on the tire and fender.
- With all 4 of these marks lined up, start turning the tire/wheel and counting revolutions, as well as the revolutions made by the driveshaft (this is easier to do with two people, for obvious reasons). At some point, both pairs of alignment marks are going to be in perfect alignment again. This happens when the tire has gone exactly twice as many revolutions as the pinion gear has teeth (usually 9 or 10), and the driveshaft has gone as many revolutions as the ring gear has teeth.
For example, if you have the typical 4.10 rear end, you would have 10 teeth on the pinion gear and 41 teeth on the ring gear. Given the way an open differential works (with one wheel held stationary), the actual rotations required to bring the alignment marks back to their starting points will end up being 20 wheel revs and 41 driveshaft revs. So... 20 divided by 2 equals 10, then 10 divided into 41 equals 4.10.
If you have a limited slip or locked differential, jack up both rear wheels instead of one, and forget the divide-by-two part of the routine above... both rear wheels will be turning the same direction at the same speed, and the alignment marks will re-align after as many turns of the tire/wheel as there are teeth on the pinion.
If this sounds complicated, it's probably because I got carried away with the detail... it is actually very quick and simple to do.
Gary L
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1971 142E ITB racer, 1973 1800ES, 2002 S60 T5
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