5-10%, definitely not. Wrong tire size will do that though.

Have had a GPS speedo on the dash for a good dozen cars through the years and never seen more than a few mph off on a flat surface. That's something I actually noticed and looked for, for some reason. A few mph over speed is fairly common, but we're talking 1-2% at most. (confirmed with Tesla Model 3, Corolla, Fusion, Prius, Elantra, Mirage, etc etc).

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Huppie 4 days ago | parent | next [-]

I know it sounds like a lot but in my experience the difference is mostly a fixed offset plus a tiny percentage due to tire pressure/size.

A fixed 2mph difference at 20mph is 10% so imho they're at least _technically correct_.

	▲	madog 4 days ago \| parent [-]
		5% at 70 mph is 3.5 mph 10% at 30 mph is 3 mph I saw this with various European cars. My experience is that it seems to be a fixed percentage rather than a fixed amount i.e. the absolute difference increases with speed.

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4 days ago | parent | prev [-]

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