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The Need for Speed!

vehicle speedometers

How accurate is GPS speed? What about my vehicle’s speedometer? Surely, a speedometer is more accurate than a GPS, isn’t it?

GPS Speed vs. Vehicle Speedometer

With a clear view of the sky, GPS speed has shown to be more accurate than most vehicle speedometers. “GPS isn’t perfect, but it’s a lot more accurate than a vehicle speedometer,” said Richard Langley, professor of geodesy and precision navigation at the University of New Brunswick, in an article for the Canadian Globe and Mail. GPS accuracy, however, may vary slightly as the vehicle travels from regions with a good view of the sky to those without. Heavily tree-lined streets and urban canyons are the biggest culprits when it comes to GPS inaccuracy, in addition to tunnels and covered parking lots.

Your vehicle speedometer accuracy can vary based on several factors, most notably differences in wheel size due to wear, pressure, and temperature. It is also typical for many motor vehicle manufacturers to have their vehicle speedometers overreport the speed to ensure that if there are errors from the sources mentioned above, you are still unlikely to be exceeding the speed limit if the vehicle states as such. “Industry officials have said speedometers are set ‘optimistically’ to give drivers an inflated sense of speed (and help them avoid tickets),” says Wall Street Journal columnist Jonathan Welsh.

What Do We See in the Data?

In the past, we have seen erroneous jumps in speed when GPS accuracy varies, as the vehicle moves into areas of reduced signal strength. The most common examples seen are speed spikes of a single second as a vehicle travels under a bridge or overpass. Subtle jumps in position while stopped (GPS wobble) can cause low speeds to appear when the vehicle should be stationary.

How Do We Ensure Accuracy?

Geotab goes to great lengths to improve the accuracy of the speed being received from the GPS. We use GPS settings such as dilution of precision (DOP) masks and GPS reported speed accuracy estimates to invalidate data. We only consider data valid if there are a sufficient number of satellites being used and the changes in speed between subsequent GPS readings are realistic. Geotab also defers to the engine-based road speed if the GPS speed is invalid or the vehicle is stationary to counter GPS wobble.

GPS is not an exact science, and we still see the odd invalid speed making it through the GPS’s, as well as our validity checks. Because of this we maintain that any speeding rules need to see multiple seconds worth of data above a given speed limit.

What’s Next?

Geotab is continuously enhancing its validity algorithms and we are currently working on further enhancing the GPS speed accuracy checks through a combination of accelerometer data, engine-based road speed, and GPS data.

If you have a concern about the accuracy of a particular GPS event recorded by your GO device or want to know more about what Geotab is doing around GPS accuracy and validity, please contact us or leave a question in the comments box below!

3 Comments

  • Posted September 21, 2016 at 10:28 am | Permalink

    Why is it that this system would report a driver for speeding if it needs to see multiple seconds and the incident only happens for a distance of .02 miles? That would be less than 1 second at 78 mph.

    • Posted September 21, 2016 at 4:11 pm | Permalink

      Please can you open a ticket with our helpdesk giving details of the event in question.

      Thanks

  • Posted October 31, 2015 at 8:31 am | Permalink

    Fleet managers often change tire brands or specs for a less expensive tires or retreads. The issue is that the amount of tread on tires varies which impacts the number of rotations used to calculate speed and distance. Some tires start with an inch of tread but after a lot of driving they get down to a 1/4″. This changes the tires circumference which reduces the accuracy of speed and odometer. GPS data may be momentarily inaccurate but these mechanical variables are far more consistently inaccurate.

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