How to check speedometer calibration
The Gearing Test
- Disconnect flexible drive from speedometer head.
- With gears in neutral, count the number of turns of inner shaft for six turns of the rear wheels when vehicle is moved forward in a straight line. (N)
- Measure the rolling radius of the rear wheels with tyres at correct pressure – centre of hub to ground. (R) (in inches)
- Apply figures to this formula: (1680 x N) / R = TPM No.