Private passenger vehicles generally adhere to the international standard of never reading below the actual speed of the vehicle but the indicated speed must not exceed 110% of the actual speed. Aftermarket modifications, such as different tire and wheel sizes or different differential gearing, can cause speedometer inaccuracy. ![]() This is done to ensure that their speedometers never indicate a speed lower than the actual speed of the vehicle, in an attempt to ensure they are not liable for drivers violating speed limits.įederal standards in the United States allow a maximum 5 mph error (plus-or-minus) at a speed of 50 mph on speedometer readings for commercial vehicles (CFR-393.82). Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error. This speedometer error is due mainly to variations in tire diameter, tire wear, tire temperature, pressure and vehicle load. That argument never works though, because by design the vehicle’s speedometer is accurate by law, or international agreement to about plus-or-minus 10%. We inform him of the fact that we couldn’t be speeding because our speedometer indicated that we were doing exactly what the posted speed limit is. Many of us “gearheads” have had the experience of being pulled over by our friendly local law-enforcement officer and being told that we were exceeding the posted speed limit. ![]() ![]() Story & Photos By Jim Clark (The Hot Rod MD)
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