GPS devices are positional speedometers, based on how far the receiver has moved since the last measurement. Its speed calculations are not subject to the same sources of error as the vehicle's speedometer (wheel size, transmission/drive ratios). GPS speed calculations generally are more accurate than a vehicleâs speedometer since it is not affected by the same inaccuracies, including the vehicleâs wheel size or drive ratios .GPS calculations are dependent, however, on satellite signal quality â although with the use of moving average calculations, any errors are considered minimal.\Vehicle speedometers also require calibration to maintain accuracy because general wear and tear, changing wheel sizes, and the manufacturerâs âerring on the side of cautionâ .Sources of error due to tire diameter variations are wear, temperature, pressure, vehicle load, and nominal tire size. Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error, to ensure that their speedometers never indicate a lower speed than the actual speed of the vehicle and setting the speedometer to read higher than the actual speed â all contribute to inaccuracies. including the ratios of the gears in the drive cable, the final drive ratio in the differential and the diameter of the tires. ..
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also read about speedometers error on wikipedia ...
