At what speed you read 10mph faster?

I think it is by law that speedo meter has to read faster than actual speed in Canada, though I can not find reference to back it up. I recall reading an article on C&D, the test result of speedometer accuracy shows all cars read faster than their actual speed with Bimmer being the extreme, read almost 10% faster.

Also you read higher speed if your tires is smaller than stock tires.

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99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.