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GPS Mileage Explained…
Why your odometer / hubometer and the mileage on your screen don’t match…
Many GPS devices display your current mileage based on a simple calculation of how much distance was covered. These readings are initially set at the time of installation and your vehicle odometer will be added to our software. It must be noted however that over time there will generally be a variation between the mileage shown on the vehicle's odometer or hubometer and the mileage showing on the report screen. This will happen with any GPS device.
How does a GPS device calculate speed / distance?
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.
Why does the GPS odometer read differ from the vehicle’s own odometer?
Variances on the vehicle;
GPS odometer calculations are more accurate than a vehicle’s odometer since they are not affected by the same inaccuracies such as the vehicle’s wheel size, tyre psi or drive ratios etc.
Vehicle manufacturer tolerance ranges;
Generally most manufacturers claim a 16m per kilometre tolerance with odometer readings, however some go as far to say a 80m per kilometre inaccuracy or more is possible. This means that over only 10,000 kilometres traveled, the manufacturer of the vehicle actually expects odometer inaccuracies of up to 160km or even up to 804km! This discrepancy means that the exact odometer reading the GPS device provides will differ from the inaccurate odometer on the vehicle.
Vehicle being under cover, between tall buildings, in canyons etc;
The GPS calculation is dependent, however, on GPS satellite signal quality (i.e vehicles may be unable to get signal accuracy due to travelling through underground tunnels, moving inside or under the eaves of buildings or similar structures or between tall buildings. These may all interrupt GPS signal) though with the use of moving average calculations errors are minimal.
Human interference;
Power loss, tampering or jump starting a vehicle can also interrupt the ability for accurate GPS odometer display.
Vehicle odometers and hubometers require calibration to maintain accuracy as general wear and tear, changing wheel sizes, the manufacturers own ‘erring on the side of caution’ and setting the speedometer to read higher than the actual distance traveled all contribute to inaccuracies.
Which odometer should a driver use?
While logged GPS data can be used to calculate notifications on software alerts for RUC and servicing it is important that the driver checks their vehicle’s recorded mileage information against the recorded GPS mileage on a regular basis. As the operator of the vehicle it is their responsibility to ascertain the information is correct before operation
We highly advise and recommend fleet owners and operators of any GPS software using this information as a tool for notification for servicing alerts and RUC alerts based on mileage both on and off road, should do diligent checks on their vehicle fleets on a regular basis to avoid any large discrepancies and every 3 to 6 months to re-calibrate the mileage for their fleets. Re-calibration is an important function with all GPS tracking due to the discrepancies that can happen as described above.
Recalibration can be done in Fleet Servicing or in Admin, the articles explaining how are linked below.