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If you see a big difference in speed readings between the app and car's speedometer or another app, then speed units may be wrong, please use the speed units switch (a "km/h" button in top-center). Every tap on the button switches between km/h->mph->knots.
If speed units are right and there is still a difference.
Car's speedometer calibration and tire size are often factors why car's speedometer would show a speed different speed from actual.
Car's speedometer calibration and tire size are often factors why car's speedometer would show a speed different speed from actual.
On the other side as GPS speedometers measure speed by distance traveled divided by time between samples (iPhone and 3G iPad built in GPS gives 1 sample per second), there is a certain delay in what GPS speedometer shows. Though when you don't accelerate or brake intensively GPS speedometer should be quite accurate.
You can read more about this subject of speed discrepancy from the trusted source: www.thecarexpert.co.uk - How accurate is a car speedometer.
Also note that EU legislation forces car makers to calibrate speedometer to show about 10% more than actual speed is. You can read more about it here.
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