Ask A Trooper

by Sgt. Neil Dickenson, Minnesota State Patrol

Question: What would be more accurate, my vehicles speedometer or a GPS I have on my dashboard?

Answer: They both can be accurate and inaccurate.

Speedometer: Aftermarket modifications, such as different tire and wheel sizes or a change in the differential gearing, can cause speedometer inaccuracy.

GPS: These 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). Instead, the GPS’s positional accuracy, and therefore the accuracy of its calculated speed, is dependent on the satellite signal quality at the time. Speed calculations will be more accurate at higher speeds, when the ratio of positional error to positional change is lower. The GPS software may also use a moving average calculation to reduce error.The content you are trying to access is only available to members. Sorry.

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