True Air Speed

M. Glodek · #1 (**permalink**) 07-14-2002, 03:56 PM

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.

Mr.Fixit · #2 (**permalink**) 07-15-2002, 10:50 AM

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.

Bob Putnam · #3 (**permalink**) 07-15-2002, 11:46 AM

ERA's speedo gear chart

I suggest you do a timed measured mile and/or compare your odometer to a real measured mile first.