Your speedometer might be lying

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According to the website TyreSize Calculator…

If you install tyres on your vehicle, with a diameter larger than your original equipment tyres have, the wheel with the new, bigger tyre, with each revolution, will cover greater distance as compared to the original tyre, for the same time interval.

Read more about the physics of the problem and use a calculator to work out your real speed if you use non-spec tyres here.