Still pretty sure the percentage will work, that's why I asked at what (exact) indicated speed is he 5mph off.
If he is also showing 5pmh at rest the speedo/odometer is suspect.
If it is working correctly except being driven at the wrong ratio given tire size and axle gears the percent error should work.
Measured mile is the better way to figure because the odometer is geared to the transmission output, no slip. The mph indicator is just an indicator, not as accurate as the miles and the thenths ticking a away.
But Denny didn't offer this info, or hasn't yet.
