Correct tire size question

[PaulE]

Originally Posted by wrs986

This car was so well cared for but the previous owner was not a “car guy” and I figured the tires would need to be replaced ASAP.
How far off will my speedometer be if I go from 265/40/18 to 265/35/18?
Right now it’s within 1 mph of my GPS. Is the speedometer reading taken from the rear or front tires? I’ll run down to tire rack next week and replace the Potenzas. What was oem for this car?

The speedometer reading comes from averaging the abs speed sensors from all 4 wheels I believe. I read it on the internet so it must be true! If you use the stock 18 inch tire sizes your speedometer will read about 2-3 mph higher than what a mobile phone GPS speedometer app is showing. There is an EU regulation or German law that fines auto manufacturers for all cars they sell whose speedometers read lower than actual speed. We've owned 1 Audi, 3 BMW's and 1 Porsche and that's always been our speedometer experience with German cars. And then there is a US law that requires all auto manufacturers to have accurate odometers so the warranties don't run out sooner than they should. So with the right sized tires your speedometer is going to read a little higher than actual speed and your odometer is going to be accurate.