We often take for granted the accuracy of speedometers. This can be a dangerous and illegal mistake. Taking your speedometer to an instrumentation shop for calibration can be difficult, expensive and impractical. The following article provides a method to test the accuracy of your speedometer without removing it from the instrument panel.
Find a level length of highway with a marked mile.
Once you have reached a given speed, engage your cruise control.
Use a stopwatch to determine the number of seconds it takes for your vehicle to travel a marked mile over that level length of highway.
Divide the number of seconds it took to drive the mile into 3,600, the number of seconds in an hour. This number is your actual mile per hour.
Compare your actual mile per hour number to the speed your speedometer indicated. If the indicated speed is higher than the actual speed, the speedometer is fast. If the indicated speed is less, the speedometer is slow.
