As it stands, it looks like the inaccuracy increases with the speed.
Johnny Bohmer Proving Grounds was out to, err, prove something in its latest top speed run video. The car in question was a stock 2017 McLaren 570S, a twin-turbo supercar that makes as much as 562 bhp (419 kilowatts) and 443 pound-feet (601 Newton-metres) of torque.
But this wasn't about how fast this old McLaren could go. They went out to prove the inaccuracy of the car's speedometer.
To measure that, Johnny Bohmer used three methods to measure speed: the car's default speedometer, a Garmin VIRB GPS, and a Tag Heuer timing equipment used by the International Mile Racing Association.
The result? At 217 miles per hour (349 kilometres per hour) on the McLaren's speedometer, the Garmin recorded 205 mph (330 km/h) while the IMRA tallied 205.596 mph (331 km/h) – a 12-mph (19 km/h) difference. It's also worth noting that as the speed increases, the gap between the GPS reading and the speedometer increases, as seen in the video above.
Now, this shouldn't be a surprise, really. It has been long known that a car's speedometer is inaccurate. If this is the first time you've heard this statement, well, I hate to break it to you but it's true. But before you call your dealer to tell him that you're returning your car because of what you've discovered, you need to stop and read along first.
Inaccuracies in speedometer readings are just natural and can be caused by many factors. To understand that, you need to know how speedometers work. In modern cars, the speedometers electronically read speed via a sensor in the transmission.
With that, variations in tyre sizes and inflation levels are the likely source of inaccuracies in speedometer readings. These aren't illegal, by the way, as manufacturers are given percentage margins of inaccuracy, and of course, as the speed increases, the inaccuracy increases, too.
Now you can ask anyone boasting about their fastest run: Is it via GPS or the car's speedometer?