WebCars! Speedometer Accuracy Test
To check the accuracy of your speedometer, you will need a measured mile. They can be found at various points on our highway system. To check your calibration, you will need to be able to drive betwwen the measured markers at a constant speed. Using the cruise control, if your vehicle is so equipped, is the best way to maintain a constant speed. You will also need to time, as accurately as possible, how long it takes to travel the measured distance. The stopwatch function found on many digital watches will work well.
Speedometers delivered from the factory can be inaccurate. Some cynics complain that manufacturers purposely calibrate speedometers to read high so that the odometers (whose calibration is related) will read high resulting in a shorter warranty! We don't have any first-hand knowlege of such a practice.
Changing tire size from what the car was delivered with is a common reason for speedometers to be out of calibration. A speedometer shop can recalibrate a speedometer by changing the its intrnal gears using the results from the form below.