Charon
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Odd as it sounds, both Serobot and I could be right. The larger tire will make the speedometer (and odometer) read lower when travelling at the same road speed. If the speedometer started off by reading high, then the resulting lower reading might be more accurate. Note that the speedometer and the odometer, even though housed in the same case and driven by the same cable, are two separate and independent instruments. It is very common for them to have different errors, and actually fairly unusual for them both to be right. The speedometer measures how fast its drive cable is turning, and the odometer counts how many times it has turned.
GPS does not actually measure speed. It measures and calculates its position on the Earth's surface. It repeats the measurement fairly often. It calculates the difference in position between successive measurements, and knowing the times of the measurements, it calculates a speed. Because there is a small error in position measurements, it "filters" its results to smooth the readings. Because a finite amount of time is required to measure, calculate, and display results, a GPS always lags by a short time. GPS gives excellent results when you are "straight and level," but if you are accelerating or braking, or turning sharply, the GPS unit's filtering might give odd results for a moment.
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