Rear end readings

If you think you'r speedo is wrong and you're not sure what your rear end ratio is, there is an easier (and far less messy) way to determine what gears you have.

Put the rear of the car up on jack stands. Put the car in Nuetral. Pick a place on the tire that's easy to identify (valve stem works well). Rotate that point closest to the ground. Now take a piece of chalk and put a horzontal line on the flange of the driveshaft that lines up with horizonal line on the rear end housing.

Rotate the tire one full revolution, while counting how many times the two horizontal lines cross paths. You will most likely see 3 rotations plus a partial. You can estimate and be pretty accurate.

You will most likely have one of these 2.73, 3.08, 3.27, 3.55, or 3.73 gear ratios.

You can check your speedometer like this.

Let's say you think it's a 3.55 gear.

Go out and drive the car down a back road in 4th gear at a steady 2000 rpm. Make note of what the speedometer reads for mph.

multiply 2000 times the measured height of your tires. Divide that by your estimated gear ratio times 336.

My car, for example would look like this: (2000 rpm x 26" Tire) / (3.73 gears x 336) = 41.49 mph

Now if the speedometer is wrong, we would have to calculate using the correct variables to figure out what gear is right for your car.

You need to know...
Tire Height
Year of the car
Year of the transmission in the car (if other than stock)
Gear ratio

There is a formula to plug all that into in the FRPP catalog that will tell you what speedo gear you need to correct your speedometer. It's simpler than it sounds.