Originally Posted by Autumn Drive
if you change your tire size do you have to change your cars computer?
Tell me you're joking. Please. :eek3:
No, you do not have to replace the ECU.
The ECU doesn't give a damn what size your wheels actually are. To calculate your speed in MPH, the ECU just counts axle RPM based on the original wheel / tire circumference. It knows that in one revolution of the axles, the car moves forward by a specific amount.
So, if the rim gets larger, the sidewalls of the tire have to get smaller to maintain the same circumference as the original wheels.
But if the circumference is larger than stock, the speedometer will read low because the wheel RPM is lower. Conversely, with an undersized circumference, the speedo will read high. Just remember that at the same speed a bigger wheel rotates slower than a smaller wheel. :run:
The easiest way to avoid this problem is to buy the correct tire size for your rim diameter. :eek5r: