Pro70z28 is right. That is the fastest way to determine your rear end ratio.

Another way to determine the ratio is to "back" your way into the number. There are only five variables to know. If you know any 4 out of 5, four then the fifth is easy to determine by simple math.

1. Transmission
2. Tire Size
3. Speed
4. RPMs
5. Rear End Ratio

So, If you know:

1. Tire diameter...(measure your tire height to determine the diameter).

2. Your transmission Gear Ratio. (If you know your transmission, we can determine the transmission gear ratio).

3. Your Speed...[If your speedometer works (or have a GPS...or someone can drive next to you at speed), then your speed is identifiable].

4. Your RPMs (no explanation needed).

We can determine your rear end ratio using the following mathmatics if you find math easier than jacking up the car and recording the rotations of the rear wheels versus the rotations of the driveshaft...

(RPM)=168 x (TR) x (MPH) / Divided by 1/2 of Tire Diameter

(MPH) = (RPM) x 1/2 of Tire Diameter / Divided by 168, then Divided by (TR)

(Ratio) = (RPM) x1/2 of Tire Diameter / Divided by 168, then Divided by (MPH)

(TR) = Rear Gear X Final Transmission Gear


I created an Excel spreadsheet that calculates this for you if you know the above input information. Let me know if you need it and I will email it to you.