On a mechanical speedometer (especially an older one) I usually figure it this way.

1. Pull the speedometer gear and get a tooth count. If you don’t have a gear, put a speedometer gear in it (it really doesn’t matter how many teeth) but you do need to know the number of teeth.

2. Take it for a test drive (ideally at highway speeds) with either a GPS or a follow car with an accurate speedometer.

3. Once you have established what the real vehicle speed is compared to what the speedometer reads divide the small number by the larger number:

i.e. Vehicle is travelling at 65 MPH speedometer reads 72 MPH

65 divided by 72 = .902 This is the percentage the speedometer is off.

If the speedometer reads slower than the vehicle is actually travelling you need a speedometer gear with less teeth to speed up the speedometer and if it reads faster than you are going you need a gear with more teeth to slow it down.

Say the speedometer gear in the transmission has 33 teeth….you will need a gear with less teeth……..start plugging numbers in the calculator again dividing the smaller number by the larger until you end up with a percentage closest to the percentage the speedometer is of .

In this case a gear with 30 teeth divided by the 33 teeth (of the gear in the transmission) gets you closest with .909 percentage.

The key is to always divide the smaller numbers by the larger to get the percentage and remember which way to go on the tooth count (more teeth to slow the speedometer done and less to speed it up).


This is the best way to accurately determine what speedometer gear you need. It takes into account any inaccuracies in the speedometer and the TRUE tire height (I have seen as much as 1 ½” difference in height between different brands of say 235/70/15s depending on who manufactures the tire and what rims they are mounted on). Additionally the 3 variables you need are the easiest to obtain; real speed, speedometer indicated speed and number of teeth on the speedometer gear.

On a side note the last 2 builds I did used aftermarket electronic speedometers……..man it sure is nice to calibrate the speedometer while sitting in the drivers seat and pushing a button as opposed to