Another question would also be what’s the difference between wheel speed and vehicle speed?
In: 620
Speedometers are actually measuring the RPM’s of the wheels.
Then they do a multiplication with a constant, based on the circumference of the OE tires (their size). That “calculation” can be electronic or just mechanical, it’s a proportional relation.
However, that constant is dependent on the tire size itself. If one changes the OE size, the calculation will be off.
Here is an online calculator that can show the expected difference in speeds:
[https://tiresize.com/calculator/](https://tiresize.com/calculator/)
A phone’s GPS will show the exact speed at every time because is based on the speed calculated versus some very precise satellites.
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