Originally Posted by GTRFAN

It's because the car travels less distance and speed than indicated.

Am I the only geek who studied physics? I am bored at work, so here goes my explaination.



Imagine the entire road surface of the wheel as distance measurement of approximately 2m. (circumference)

The ECU is constantly counting how many times the wheel rotates and how many times per second this occurs via the ABS wheel sensors (or gearbox on some cars)

The ECU then multiplies the distance by the number of times and gets the total distance travelled. It can also calculate speed based on the number of rotations per second/minute/hour

Here's the problem....the ECU has a distance measurement for the wheels that it uses for all calculations. This figure is the innacurate part and is a bit longer than actual wheel circumference.