Gear Ratio Help

I just purchase an xj that has already had some modifications done to it. It currently is running 33/12.50's. I was wondering if there was a way to check its gear ratio by the RPM vs speed. The speedo has not been changed, but it shows in 3rd at 2500 running 55mph (which is probably closer to 58-60 with the tires). Anyone want to throw out a guess of what the gear ratio might be? Thanks.

Heh.... no bets here.

I'd block the XJ, and jack up the rear end. Count driveshaft rotations for 1 wheel rotation. Or, pull the rear diff cover off and count teeth.

ChiXJeff said:

Heh.... no bets here.

I'd block the XJ, and jack up the rear end. Count driveshaft rotations for 1 wheel rotation. Or, pull the rear diff cover off and count teeth.

Click to expand...

Ditto.

Unless you KNOW exactly what speedo gear is in there, there is no way to correlate RPMs to speed.

Correction -- if you use GPS or one of those police radar trailers to get an accurate speed, independent of the speedometer, then you can use tire data and engine RPM to arrive at a gear ratio. It's probably easier to jack it up and spin the tires.

Gear ratio = rpm x tire diameter /Mph x 336

Thanks for the info. I will plan on lifting up the rear this weekend and seeing how that number compares to the formula. It is great to be back on . Now I just have to get the membership transfered.