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 Sacramento Valley MG Car Club

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   Norm Nock
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 How Accurate Is Your Speedometer?

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.

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Norman Nock
British Car Specialists, Stockton, CA