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by Fade_Dance 284 days ago
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).
1 comments
Huppie 284 days ago
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_.

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madog 284 days ago
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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