The Inaccuracies of Speedometers

Many times a driver is given 30 minutes to get to a destination that requires 45 minutes. So, at times, there is a tendency to drive a little faster than the speed limit allows. One of the questions we get asked is – “how accurate are speedometers”?

The effect of tire pressure and load on speedometer accuracy.
Variations in tire size and inflation levels are the sources for most errors. Normal wear and under-inflation reduce the diameter of the tire, causing it to spin faster and produce an artificially high reading. From full tread depth to baldness, speeds can vary by up to about two percent, or 1.4 mph at 70 mph. Lowering tire pressure five psi, or carrying a heavy load on the drive axle, can result in about half that difference.

How to check your speedometer
It is fairly easy to see how your speedometer compares with your actual driving speed. One way to check speedometer accuracy requires an empty, flat highway with mile markers and a speed limit above 60 miles per hour and a friend with a stopwatch. It should take you 60 Seconds to drive one mile; we suggest using cruise control if possible.

Try this a few times, getting the average and then compare the number of seconds to 60 to calculate the percent error.

GPS Vs. Vehicle Speedometer
Another way to check speedometer accuracy is to compare it to a GPS device.

From a Car and Driver magazine article.

When using GPS device to measure speed, the numbers will be different than the vehicle’s speedometer. Why and which speed is accurate? GPS speed calculations are more accurate than a vehicle’s speedometer since it is not affected by the same inaccuracies, including the vehicle’s wheel size or drive ratios.

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