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How accurate is a GPS-based speedometer?

29 Dec 2011 Jason 26

Many GPS devices display your current speed, based on a simple calculation of how much distance you are covering in a given time period. However, drivers are often confused with these readings since it generally varies significantly from the vehicle’s speedometer.

So which speedometer is more accurate? Which one should be used to determine if you are driving within the posted speed limits? How does it relate to using speeding alerts in your telematics program?

Speeding is widely regarded as a significant contributor to fatal road accidents, and fleet owners find GPS-based speed calculations very useful in maintaining fleet safety. It’s important then that these speed calculations can be trusted to avoid drivers being falsely accused of speeding, as well as providing a reliable guide to determining a legally-safe driving speed.

How does a GPS device calculate speed?

Using GPS, a device is able to calculate a lot of information about a moving object. Using even basic time and location data, a GPS unit can quickly calculate the relative speed of the object, based on how much distance it covered in a given time.

GPS devices are positional speedometers, based on how far the device has moved since the last measurement. The algorithm also uses the doppler shift in the pseudo range signals from the satellites. It should also be noted that the speed reading is normalized, and is not an instant speed.

Speeds are updated at short intervals to maintain accuracy at all times. It uses frequent calculations to determine the vehicle’s speed. For example, using a standard movement per time calculation, if you have covered 80 feet in one second, the GPS device works out and converts that to MPH, which in this case is 55MPH.

Why does GPS speed differ from the vehicle’s own speedometer?

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. It is dependent however on GPS satellite signal quality but with the use of moving average calculations any errors are minimal.

Vehicle speedometers also require calibration to maintain perfect accuracy as general wear and tear, changing wheel sizes, and the manufacturers own ‘erring

on the side of caution’ and setting the speedometer to read higher than the actual speed, all contribute to inaccuracies.

Generally most manufacturers claim a 0.1MPH tolerance with speed readings, however will go as far to say a 0.5MPH inaccuracy is possible, to make allowances for any satellite signal variations beyond their control.

Which speedometer should a driver use?

It really comes down to personal preference since both gauges are reliable for determining a safe, legal traveling speed. In most cases the vehicle’s speedometer will show a higher speed due to the aforementioned inaccuracies but this at least ensures the driver stays lower than the posted speed limit.

And while logged GPS data has been used to overturn speeding tickets, the cautious driver, who is keen to avoid infringements, fines, trouble from their employer or the possibility of an accident, will tend to err on the side of driving slower rather than pushing it too close to the limit.

Using GPS to record company speeding incidents

Any business that has company vehicles available for employee use is no doubt keen to avoid dangerous speeding, both from a public liability point of view, the possibility of accidents as well as infringement notices or demerit points. Using fleet management software such as Telogis Fleet, fleet owners are easily able to specify speeding notifications that can alert managers to speeding incidents in real time, or provide regular reports of this sort of activity.

Speeding alerts can be setup to record either speeds in excess of the posted speed limit, or simply if a certain speed is reached (ideal for vehicles that become unsafe above certain speeds e.g. forklifts).

How closely the speeding alerts are monitored can be set as well, such as how long the vehicle is speeding before the alert is triggered. This can be used to avoid false positive reports due to overtaking or temporary distraction.

Since employers know that speeds calculated by GPS devices are not only more accurate than a vehicle’s speedometer but also lower, it gives employees no excuse to be caught driving at unsafe speeds. Through a combination of regular reporting and driver coaching, fleet owners can make sure they are doing everything in their power to improve the safety of their staff and other road users.


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