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Message: <blockquote data-quote="seech" data-source="post: 363762" data-attributes="member: 4572">bimmerdude, if i'm not mistaken, all car manufacturers have speedos which is higher than the actual speed itself. apparently this is for legal reasons (imagine if it is lower than your actual speed and you get saman etc. etc.). The only one that was accurate was for the McLaren F1 which was spot on when tested by Autocar.Best way to test is to measure the km marker post when on the highway. If you are travelling at 120kmh, 1 km would take 30 seconds. So far most of the cars that I have driven under report by around 10 kmh. Changing rims and tires also alters the rolling radius of the tires and will have an effect on actual and reported speeds. Before I changed to 17" mine was around 10kmh lower. Changed to 225/45 was around 5km, now with 235/45 its accurate.Don't agree with the answer given that odometer, mpg, etc are not effected. What rubbish. My fuel consumption has 'dropped' as I now cover less reported km due to the tire size difference.Just my 2 sen.</blockquote>