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Message: <blockquote data-quote="fabianyee" data-source="post: 363803" data-attributes="member: 6">please first understand how the car gets its speed reading... It gets the reading from the pulse generator at the rear differential.... This has been the way for all BMWs for the past 20 years. i believed this is still the same for the current models. It calculates the number of rotation per sec/minute of the driveshaft (output flange), then the speedometer would translate that to its speedo reading based on the stock tyre size hardcoded into the calibration.. So if you change the tyre size which has a LARGER diameter or circumferrence, it would definitely affect the speedo reading. And it doesn't have a second system to read the speed, unless it uses the in-car GPS system to countercheck the reading... And yes, almost all manufacturer built in higher speed reading between 5-10kph...</blockquote>