|Here is an easy way to
check your speedometer. Road test your car on the highway to check the odometer's accuracy
against the speedometer highway markers.
Check the TPM (turns per mile) number on your speedometer. The number is normally in the
center of the gauge (i.e. 980, 100 1 1325). The TPM number shows the amount of times that
the cable will rotate in one mile. See figure 1.
To check how many TPMs your cable is doing you must first mark 52.80 feet (1 / 100 of a
mile) on a level surface such as a garage floor or a driveway. Remove the speedometer
cable from the rear of the speedometer unit. Fit a cardboard counter on the inner flex of
your speedometer cable as shown in figure 2. Count the number of turns the counter makes
when you push the car the marked distance of 52.80 feet (52 feet. 9 1/2 inches). Then
multiply the number of turns by 100. This number should equal the TPM number marked on
your speedometer face plate.
If the TPM number does not equal the turns of the cable, your problem could be:
Incorrect Speedometer gauge for vehicle Incorrect tire size
Incorrect size wheels
Incorrect rear axle ratio
Fault at cable drive end
Drive that is on/or to the
main shaft is faulty
Check for a kink in the inner flexible shaft by rolling it on a clean flat surface. Kinks
can be seen and felt as shown in figure 3.
British Car Specialists, Stockton, CA