Calibrating Odometers and Speedometers

This is the formula used for calibrating an Odometer and Speedometer to a vehicle.
I have used this little formula for many years and find it quite accurate, but I still like the ultimate test for speedo accuracy by using a GPS.

1680xN Divide by R = T.P.M. No

Were N= Number of turns of the inner shaft of the speedo cable for 6 turns of the wheel that drives the cable and R= the radius of the wheel in inches measured from the centre of the hub to the ground. Example - Cardboard pointer on inner shaft rotates 12 1/2 turns as vehicle is pushed forward 6 turns of rear wheel - I say pushed forward so as to illustrate the need to allow for the differential gearing in a car - a chalk mark on the tyre helps here also. Grandchildren can help count the rotations of the cardboard pointer - or do the pushing!

Wheel radius is 13"
Flex turns per mile =
1680 x 12.5 divide by 13 = 1616 T.P.M.
For a metric - kilometer - speedo substitute 1680 for 2653 and measure wheel radius in centimeters

Example 2653 x 12.5 divide by 33 = 1005 T.P.K. The TPM or TPK figure is on a lot of dials but not all.

- Andrew Rackstraw 10/25/02