The speedometer cable is now turning too fast, so a new driven gear in the transmission having more teeth is needed to slow down the cable. With a perfect setup, the cable makes 1000 revolutions per mile and spins at 1000 rpm when registering 60 mph.
Without knowing the actual car speed where the error is 5 mph fast, we don't know how much correction is needed. The amount of error should be proportional to the speed driven. However mechanical speedometers were never that accurate at indicating a wide range of speeds.
Instead we can calculate the amount of correction needed if the circumference of the original equipment tire is compared to that of the new tire. Another way is to drive a known distance following a newer car with an electronic speedometer and original-size tires. Then compare how many miles your car registers vs the newer vehicle.
Recently a guy here in the forums couldn't obtain replacement driven gears for his car so he had a small ratio-changing gearbox custom built to do the job. That worked out really well so he didn't have to worry about speeding tickets any more.
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