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Thread: 69 Skylark. Speedometer 5 miles an hour fast.

  1. #1
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    Cool 69 Skylark. Speedometer 5 miles an hour fast at 60 mph.

    Hi Guys,
    I have a 69 Skylark Custom Convertible. Automatic. Due to putting smaller tires on the car, my speedometer registers 5 miles an hour too fast. I know I can change the odometer drive gear, in the transmission. My question is, would I go to a larger or smaller gear? Also, how many teeth difference should I change for 5 miles per hour?
    Thanks to all. Have a great day.
    Casey
    Last edited by casey; 05-02-2017 at 06:44 AM.

  2. #2
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    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.
    What has been, can be again. (Bob Wills, 1942)

  3. #3
    is there a particular reason you're putting a shorter tire on the car?

    you can adjust tire height by getting a wider tread OR getting a larger percentage profile ( move from a 60 to a 65 for instance ) sidewall.
    The way to crush the bourgeoisie is to grind them between the millstones of taxation and inflation.
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  4. #4
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    69 Skylark. Speedometer 5 miles an hour fast.

    Quote Originally Posted by casey View Post
    Hi Guys,
    I have a 69 Skylark Custom Convertible. Automatic. Due to putting smaller tires on the car, my speedometer registers 5 miles an hour too fast. I know I can change the odometer drive gear, in the transmission. My question is, would I go to a larger or smaller gear? Also, how many teeth difference should I change for 5 miles per hour?
    Thanks to all. Have a great day.
    Casey
    Bigger

    Whatever percentage you are off, go that percentage bigger.

    Example: 50 actual mph shows 55 mph on the speedo ---> 10% difference, therefore get a gear that has 10% more teeth

  5. #5
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    Thanks to everyone

    Thanks to everyone that replied. I have this condition corrected. Enlarging the gear did the job.

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