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Ask a Trooper: Which is more accurate: a speedometer or GPS?

Q: What would be more accurate, my vehicles speedometer or a GPS I have on my dashboard? A: They both can be accurate and inaccurate. Speedometer: Aftermarket modifications, such as different tire and wheel sizes or a change in the differential g...

Sgt. Neil Dickenson
Sgt. Neil Dickenson, Minnesota State Patrol

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

A: 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.

I am able to check my speed with radar and laser but in my personal vehicles, one of my most common methods for checking the accuracy of my speed is by using a watch. Use a watch with a second hand or preferably a digital stop watch. I will explain how to go about this: On a state highway or interstate you will see mile markers (numbered green posts). Get your speed set with your speed control and when the front of your vehicle crosses the mile marker, start timing, and check the time it takes you until the front of your vehicle crosses the next mile marker. This will give you an indication of your speed. Make sure that conditions are clear and dry and that you do not have to speed up or slow down. Traffic and hills are also an issue, so use good common sense when doing this. I would take several checks in a row and do this on a regular basis. Here is a guide to serve as a reference for speed computation in one mile:

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  • 50mph = 72 sec.
  • 51mph = 70.5 sec.
  • 52mph = 69 sec.  
  • 53mph = 68 sec.
  • 54mph = 66.5 sec.
  • 55mph = 65.5 sec   
  • 56mph = 64 sec.
  • 57mph = 63 sec.  
  • 58mph = 62 sec.   
  • 59mph = 61 sec.
  • 60mph = 60 sec.  
  • 61mph = 59 sec.  
  • 62mph = 58 sec.
  • 63mph = 57 sec.
  • 64mph = 56 sec.  
  • 65mph = 55 sec.  
  • 66mph = 54.4sec.  
  • 67mph = 53.5 sec.
  • 68mph = 53 sec.
  • 69mph = 52 sec.
  • 70mph = 51.4 sec.
  • 71mph = 50.7 sec.
  • 72mph = 50 sec.
  • 73mph = 49.3 sec.

So remember, your speedometers and GPS units are not always accurate, yet you are responsible for the speed you are traveling. Make checking your speedometer a regular habit and you will be less likely to see yourself stopped along the highway and finding out the hard way your speedometer is off. Sgt. Neil Dickenson is a public information officer with the Minnesota State Patrol. Send your questions to trooper@duluthnews.com . You may remain anonymous if you wish.

Related Topics: POLICE
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