About 10 years ago Car and Driver tested various cars about the accuracy of the speedometer readings. BMW was by far the most consistently optimistic speedometer reading. I have had five BMWs since 1992. Each one has had about he same error in the speedometer. At highway speeds the speedometer is usually about 3mph over the actual speed. I think the error is a percentage, so that the faster the car is going the larger the speedo error.
2015 i8 Tera World Protonic Blue