Why GPS, OBD scanner and speedometer show different speed?

And which of them tells the truth?

With HUDWAY Drive, you can view the speed retrieved by the OBD-II scanner, or the one calculated by the app and GPS-based — while at the same time observing what your car speedo reads.

The three will differ — and here's what makes the desired unambiguity hardly attainable.

Be it a mechanical or an electronic speedo, it's a system of elements — pretty complex, vulnerable and error-prone.

Minor inaccuracies will pile atop one another, and then a few outer things will factor in. Like, the tires.

Both math logic and experiments agree that the speedo accuracy is directly and significantly influenced by the tires size (while the tire pressure change only contributes a very minor error) — as the car manufacturers calibrate their speedometers in accordance to their default tires. In case you change them, the speedometer should be recalibrated, otherwise the speed will be far from accurate.

While the two abovementioned factors may not always be in place, maintaining the indicated speed higher than the true speed of the vehicle is a must for the vehicle manufacturers — and this means, your car speedo will always read higher than you drive.

According to the United Nations Regulations, there shall be the following relationship between the speed displayed (V₁) and the true speed (V₂):

To put it simple, if the real speed is 100 km/h (~62.1 MPH), the speedo will show something between 100 km/h (~62.1 MPH) and 114 km/h (~70.8 MPH).

This may further vary depending on the country and the vehicle type, but all in all, the car manufacturers must comply with these Regulations — otherwise, their vehicles are not approved to be sold.

Once you plug a Drive-compatible Bluetooth scanner into your car's OBD-II port, the app will take the the vehicle computer's data to visualize your RPM, fuel level and consumption, etc. — and the speed.

The vehicle's computer gets the data from various scanners, which are not lab grade precise. So, expect some 5% error in the data incoming to the computer, which can further be tweaked by the vehicle software into you never know which side.

Just as with the car speedo, the speed data received via the scanner won't be error-free if you replace your default tires or made any other customizations.

To make sure it is, check that the gear ratio matches the engine control unit, the tires are stock size, the speedo gear is correct.

The GPS need to be linked to satellites to exchange signals with them and know your position and how fast it changes.

The more satellites you get at your disposal, the better positioning accuracy you get (you need at least three to position you, and one more to eliminate the errors).

However, their position is of importance, too: the more evenly they're distributed in relation to the receiver (your phone, in this case), the more accurately they're able to position you.

The official US Government information states that well-designed GPS receivers have been achieving horizontal accuracy of 9.8 ft. (3 m.) or better 95% of the time, while a regular smartphone is typically accurate to within a 16 ft. (4.9 m) radius under open sky.

Which takes us to the next point.

Any obstacles in between your phone and the satellites — be it clouds, trees, buildings, or tunnels — blocking or reflecting the signal, will create positioning errors and delays. This is why you often see degraded GPS accuracy in downtowns.

While on a straight run and if maintaining a steady speed, the error will be lower than 1%, GPS isn't that good at understanding road curves, which is why the speed calculation accuracy may fall on a winding road.

Despite these factors, GPS will typically outperform the car speedo in terms of speed definition accuracy. The studies say that as long as there's no problems with the GPS signal the speed accuracy will be within about 0,2 of a kilometer per hour.