lately my car has been showing 74mph on the speedometer @ 3000 RPMs

I used the formula

MPH= (tire dia in inches * RPMs) / (336 * diff ratio * 4th gear ratio)


this is where I found this formula

MPH =(24.8" * 3000RPMs) / (336 * 2.74 * .98)

MPH= 82.5


I made five one mile runs. timed with a stop watch. my cruise was set at 60mph on the speedometer the adverage time was 55 seconds per mile

60mph/60min = 1mile per minute @ 60mph

1 hours is 3600 seconds so 55sec / 3600sec = 0.0153 hrs

use the distance formula D=RT leaving R as the unknown R= D / T


R=1mile / 0.0153 hrs

R=65.4mph

so my speedometer is 5.4mph off. dry.gif


Percent Error = (math value - speedomete value) / speedometer value x 100%

PE = 9% off for the speedometer



percent error for the car over all

so if I go back and add 5.4mph to 74mph its 79.4mph at 3000RPMs

PE = (82.5 - 79.4) / 79.4 x 100%

PE = 3.90%

so my car is within 4% of where it should be at a certain RPM. I guess thats not soo bad on a 123000+ mile car.