How accurate is a GPS-based speedometer?
Dec
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.
September 7th, 2012 at 12:31 pm
I ride a motorcycle and when I use my GPS the speed differential between speedometer and GPS is quite significant. I decided to calculate the speed according to the RPM of the engine adjusted by all the different gearing ratios to determine the rear wheel speed based on the actual circumference of the wheel. According to these calculations my speedometer is accurate making the GPS wrong. If the mathematical calculations say the speedometer is accurate why does the GPS differ so significantly? TYre sizes are standard according to the manufacturer’s specifications as well.
October 24th, 2012 at 10:06 am
Seed is proportional to the distance. Since that, car manufactures intentionally calibrate higher speedometers to void earlier car warranty. Warranty is dependable of two factors: time or mileage.
October 30th, 2012 at 1:27 pm
This is almost all wrong. The GPS speed calculations based on distance between positions is quite inaccurate. Most GPS positions are accurate only to within ~5m. This means if you actually travel in a straight line of 50m in 1s, the GPS could see that as a distance of up to sqrt(50^2 + 5^2) = 50.24m. There is an accurate form of GPS speed measurement but this is using Doppler shift measurement. Have a look: http://nujournal.net/HighAccuracySpeed.pdf
November 27th, 2012 at 1:32 pm
Does a GPS speedometer just use horizontal movement. If you went down a steep hill then up the other side wouldn’t the gps only see the movement as the distance between the two peaks rather than that travelled down to the bottom and back up the other side.
January 14th, 2013 at 10:16 am
@4 Whether the GPS speed reading takes into account the movement in 3D is dependant on the software, one would hope that designers had taken this into account.
@1 Motor cycle – You do not know the accuracy of your speedo or rpm guages. They have been set with assumptions on wheel diameter and general accuracy and legal considerations. GPS IMO gives a better result.
@3 – GPS accuracy. It is well known that GPS has an absolute accuracy of +/-5m, this is mainly dependant on timing accuracy which is fairly fixed over short period, therefore relative accuracy is higher. Furthermore, even considering GPS using absolute errors as a worst case, you can see that it will get more accurate as the speed increases as the position error becomes less significant over the distance between samples increases.
Generally I find my speedo and GPS differential fairly linear as a %. Since I know the GPS is more accurate higher up, I consider that both are fairly close lower down and the absolute differential is small in both cases. The speedo error seems mainly down to the fact that the designer has built in a ‘always read too high worst case’ into the car so they do not get sued for speed infringements/injury by owners rather than choosing the most accurate reading over the expect range of rolling radius’. ie if the system can manage +/-2.5% they design the speedo to read +5%.
March 16th, 2013 at 11:32 am
I think SBurns question deserves more specifics. Your statement “@4 Whether the GPS speed reading takes into account the movement in 3D is dependant on the software, one would hope that designers had taken this into account.” Is not a good answer since you include the word “hope”. Can you name GPS manufacturers that include software/algorythms that truly calculate speeds in 3D? Other than time and distance, what is the third variable used by GPS satellites to accomplish 3D speed calculations?
Also, what is the refresh rate of the speed calculation? It can’t be in real time regardless of how fast the refresh rate is since “distance traveled” used for calculations are always past tense.
March 16th, 2013 at 11:54 am
One more comment. If you have a speedometer checked by a device that is calibrated to a NIST standard, the resulting readings will reveal the true and actual speedometer display while taking the total possible error variables into account (i.e., at a true 50mph the speedometer displays 48 mph, at a true 60 mph, the speedometer displays 59mph, etc.) After all, the wheels on the road travel the complete distance by marrying itself to the asphalt during the distance traveled. Obviously, there is no need for a third variable to calculate speed or distance….and it’s real time.
If you know your speedometer certified error, by interpolation your speedometer reading is the most reliable instrument to keep you accurately informed of your speed.
March 21st, 2013 at 1:01 pm
@5 TonyS – You and everyone else ignores my question which is “The arithmetical calculation of the speed of my motorcycle equals the speed shown on my speedometer which differs from the speed shown on my GPS. WHY???”. I have used the actual gear ratios as published by the manufacturer. I calculated the circumference of the driven rear wheel AND I actually measured that circumference. You later comment that your GPS and speedometer differential is a fairly constant %, therefore the GPS must be correct. I could just as easily say the speedometer must be correct. Your statement is merely your supposition as you have no evidence proving it’s veracity. No one here has given any real explanation to my question. Everybody just says the GPS must be correct. I just rode 315km from Sedgefield to Port Elizabeth based on my odometer. When my GPS calculated the distance it said it was 288km. I often find that the GPS distance also differs from the ditance shown on signboards along the road but my odometer always agrees with those signs. How do you explain that?