Appendix 6 Speedo Calibration
Spe
edometers 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 speedometer readings 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...
(1) Take the measurement from the centre of the hub of a drive wheel to the ground, with the tyre
pumped to normal pressure.
ANSWER:(1)__________ distance in Inches
(2) 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
ANSWER:(2) __________ distance in Inches
(3) 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 RESULTS *
When you have completed the calibration information sheet, send it and your gauge to the address below
with a cover letter stating that you have built your own Suffolk SS100 and you require an express return
service.
SPEEDY CABLES LTD.
ABERCRAVE
SWANSEA
SA9 1SQ
01639 732213732299 Fax: +32238
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