Odometer question

[Chamberpains]

When your tire revolutions are off or you change your gears ratio and the ECM doesn't know it, it causes a linear variance. In this case it would be when you're going say 5 miles an hour the speedometer is reading real close to the correct traveled miles per hour. As you increase your speed say to 30 miles an hour you begin to see a 2 mile an hour or so difference from the true speed travel. As you get up into highway speeds the gap gets even larger which is where you're going to get you're 4 to 8 mile an hour difference from your true rate of speed.

Its important to the speedometer that it knows your correct rear gear ratio, tire size and tailshaft output ratio in the highest gear of the transmission or sometimes how many teeth are on the transmission tailshaft speedo gear so the speed sensor can relay the correct information for the ECM to calculate your rate of speed.