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True Air Speed
Have a Contemporary Cobra, 428, toploader, and 3:31 Jag rear. Speedometer seems to indicate 1/2 actual speed. How do you determine the correct size speedometer gear so the speedo is indicating the true air speed? Somebody said there's a website available.
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A toploader has a 1:1 fourth gear. Your diff as a 3.31:1 gear ratio. Measure tire size (circumference). You should now be able to calculate how fast you are going based on RPM.
Assume 28" tire?: 3000 rpm divide by 3.31 is 906.3 tire RPM 28" tire has a circumference of 7.33' so, 906.3 multiplied by 7.33' is 75 miles per hour You would be going 75 mph in fourth gear at 3000 rpm if you have a 28 rear tire. |
ERA's speedo gear chart
I suggest you do a timed measured mile and/or compare your odometer to a real measured mile first. |
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