Guidelines for Speedometer Calibration
Speedometers cannot just be changed from one vehicle to another. They only read accurately when used on the unmodified vehicle for which they were originally calibrated. Even when used on the original vehicle, any changes that may have been made to the wheels, tyres, gearbox or other related parts can seriously affect the speedometers reading to the extent that the vehicle will fail to pass an SVA test or may result in a speeding fine. In this case, the speedometer will need to be re-calibrated.
To ensure that your speedometer reads accurately when fitted to your particular vehicle, we will need to calculate the required “revolutions per mile”
For calibration of a new instrument, or re-calibration of an existing instrument, we will need you to provide the following information from your vehicle:
- Take the measurement from the centre of the hub of a drive wheel to the ground (distance measured in inches), with the tyre pumped to normal pressure.
ANSWER:(1)______________ distance in inches
- Put a chalk mark at the bottom of the measured wheel also marking on the ground where it meets. Push the vehicle forward one revolution of the chalk mark and record the distance travelled (measure in inches).
ANSWER:(2)______________ distance in inches
- Now disconnect the speedometer and place a cardboard arrow on the end of the protruding inner speedometer cable.
Put a chalk mark at the bottom of the measured wheel and then push the car straight forward (with gear in neutral), counting exactly 6 revolutions of the wheel, whilst a partner counts the number of times that the arrow on the cable revolves.
ANSWER:(3)______________ number of turns of the cable. (N.B. include parts of a turn e.g. seven and a quarter turns)
DO NOT DO THIS BY JACKING UP THE WHEEL AS THIS GIVES COMPLETELY FALSE READINGS
Now send us the values that you recorded for Answer(1), Answer(2) and Answer(3) above.
It requires a skilled instrument mechanic and the use of specialised instrumentation to calibrate the many types of speedometers which we receive from our clients.
An accurate calibration always relies heavily on the accuracy of the above measurements, which we ask you to provide with the instrument.