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How to check speedometer calibration

The Gearing Test

  1. Disconnect flexible drive from speedometer head.
  2. 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)
  3. Measure the rolling radius of the rear wheels with tyres at correct pressure – centre of hub to ground. (R) (in inches)
  4. Apply figures to this formula: (1680 x N) / R = TPM No.