Why all the driveway measurements, assumptions and speed related changes? Just
drive the car at the speed you normally drive, on a highway with mile markers.
Go at least 10 miles, the farther the better. Record the miles on the
odometer. Divide the odometer miles by the actual highway miles and multiply
by the number of teeth on your driven speedo gear. The number you get is the
number of teeth the new gear should have. Buy one as close as you can on the
low side, that will allow for tire wear.
After you install the new gear, check it out. If the mile markers come out all
right then drive 60MPH. The mile markers should go by at one per minute. If
the speed reading isn't close enough for you then the Speedometer needle
spring must be adjusted.
John Logan
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