Why is Google Maps, Waze and sat-nav speed different from my car speedometer?

Tech giant Google has just added vehicle speed to its Maps display. But why is the speed shown different from the car’s speedometer?

Rival Waze also shows a vehicle’s real-time speed. So do most portable navigation units that drivers suction to their windscreens.

Why does this happen? It’s a common question, so it’s worth revisiting for those who may not be aware.

The vehicle’s speed is calculated by the car – via the rotation of the tyres – and then car companies deliberately factor in a margin for error, so they avoid being prosecuted over speeding tickets.

Generally speaking, the speedometer on most new cars – whether shown via a digital display or an analogue dial – over-estimate your true speed by about 3kmh at 100kmh (or about 3 per cent across all speed ranges).

Over 20 years of testing we have seen some car speedometer variations of up to 6 per cent versus our precision VBox timing equipment.

It means that if you get booked for speeding when you were relying on your car’s speedometer, chances are you really were speeding.

If your car’s speedometer was showing 125kmh, your true speed detected by a police radar, lidar or speed camera would likely have been closer to 121 to 122kmh.

Police also know that most cars have a margin for error.

So when your true speed flashes up on their radar or lidar, they know you were probably looking at a speedometer showing a higher speed than what they have recorded you at.

Car makers over-estimate speed so they don’t get sued by motorists for under-calling it – and so you have a better chance to be safe, and not get nabbed.

Just in case you’re wondering, police highway patrol vehicles in most states are specially calibrated to display their true speed on a separate device – or they use super-accurate radar or lidar devices (that are tested before each shift) to detect speed.

So where does the vehicle speed on your Google, Waze or navigation unit come from?

The same satellites that pinpoint your location to give you directions.

That is why you will often see a discrepancy of 3kmh or so at 100kmh on Google, Waze or a portable navigation unit versus the speed displayed by your car.

Car makers also factor in some leeway to allow for changes in a vehicle’s tyre wear or tyre pressure over time, all of which can make a tiny difference to the vehicle speed displayed.

One thing is for certain: the GPS-based speed in the corner of your Google, Waze or portable navigation unit is much closer to your true speed – and closer to the speed that police will see if they’re pointing a radar or lidar device at you.

For the tech nerds, here are some excerpts from the Australian Motor Vehicle Regulations which car manufacturers must abide by:

The accuracy of the speedometer equipment shall be tested in accordance with the following procedure:

5.2.1. The tyres shall be one of the types normally fitted to the vehicle as defined in paragraph 2.3. of this Regulation. A test shall be carried out for each type of speedometer intended to be fitted by the manufacturer.

5.2.2. The test shall be carried out with the vehicle at its unladen weight. An additional weight can be carried for purposes of measurement. The weight of the vehicle and its distribution between the axles shall be indicated in the approval communication (see annex 1, paragraph 6.);

5.2.3. The reference temperature at the speedometer shall be 23 ± 5 °C;

5.2.4. During each test the pressure of the tyres shall be the normal running pressure as defined in paragraph 2.4.;
The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.2.5. above, there shall be the following relationship between the speed displayed (V1 ) and the true speed (V2).