Checking Gear Ratios

[Kevs68]

I have a 68 AMX. I'm assuming it's a stock 3:15 rear end, although the previous owner mentioned he thought it was a 3:54. Is there a reasonably-easy way to tell?

Thanks!!!

[donsjave]

Most of AMC rear ends are the model 20 AMC rearend. There should be a tag that is attached to the rear end that gives you a series of numbers. I believe that you take one number and divide it by the other. The tech manual tells you how I believe.
The other way is to jack up the car and get both wheels off the ground and count the rotations of the wheel you are turning verus that of the drive shaft. There is a formula that Peterson's Big Book uses and if you do not have one just let me know and I will provide the formula to you. I would do it now, however I left my book at work.

[Kevs68]

Thanks Donsjav, appreciate the tips.

I'll take that formula from Peterson's next time you get a chance. Much appreciated.

Kevs68

[donsjave]

Okay, here we go.
Mark the driveshaft or the u-joint just ahead of the differential with a grease pen and the base of the tire. Then rotate the tire slowly, either in the forward or backwards direction. While you are doing this count the the number of revolutions the driveshaft makes. Do this for 10 full turns of the tire until the mark is at the bottom center. The driveshaft will not stop in the same position as when you started, but will be somewhere in the middle of the revolution and you must determine how far it has gone on this final turn.
Judge the percentage as carefully as you can then divide the driveshaft rotations by the wheel rotation. For example, assume that driveshaft roatated 15 1/2 times while the tire went 10. Divide the 15.5 by 10 and you get 1.55, but this is only 1/2 of the axle ratio. When one wheel is stopped and the other turned, the differential operates at twice the rate it would if both wheels were turning. Thus you must mutiply 1.55 by 2 to arrive at the ratio, or 3.10 You'll find that your answer will not come exactly on one of the ratios available for your car during the particular model year, but it will be close enough so that you can determine which it is if your car has optional ratios.
If your AMX has posi on it the wheels will turn together when jack up while you are rotating the tire. In this case you will not have to mulitiply the ratio by two. Dividing the driveshaft turns by the wheel turns will give you the final ratio
Any further questions, Just ask