You won’t be alone in noticing the difference between what your speedometer displays compared to what speed your sat nav says your travelling at. Ever wondered why that is? Well we had the same thoughts, so we got a little guidance from our friends at Garmin to shed a little light…

The speedometer in a vehicle is based on calculations using tyre rotation. GPS devices calculate speed based on your GPS position. The accuracy of the speed displayed on the vehicle and GPS device can vary depending upon whether the vehicle’s speedometer needs to be calibrated and/or how accurate your GPS reception is – Garmin

The nitty gritty

The technical stuff sounds like this: your speedometer works by continuously counting turns of a driveshaft, the gearbox or a wheel, and relating them to the rolling circumference of the car’s tyres. This is the number you see displayed on your speedo.

Sat navs on the other hand, calculate your speed by constantly updating your exact location via GPS, measuring the distance traveled since your last recorded co-ordinate and comparing it to the time taken.

Accuracy

Because of the way your car calculates speed, speedometers have to allow for a margin of error. With wear, the circumference of your tyres can change, which in turn affects the recorded number on your speedo. The same goes for sat navs, which rely on GPS readings. There can be some lag in the readings due to signal quality, which is just one of the factors affecting the speed you see on your device. Bends in the road and even inclines can also affect how accurate a reading you get.

So which one is right?

In general, your sat nav is more likely to be more accurate than your car’s speedometer, however that being said, it’s better to rely on your speedo as it will never understate your speed:

GPS devices should be used as a navigational aid and should not be relied upon for precise measurement of speed – Garmin

 

3 comments

  1. Speedometers USUALLY overstate the speed ie true road speed 30, the speedo will normally show around 32/33. This gives you a margin of safety as regards exceeding a speed limit. If it’s the other way round, the speedo probably needs attention etc.

    Subject to the factors mentioned in the blog, satnavs rely on at least three satellites to triangulate speed so are more accurate.

  2. , it’s better to rely on your speedo as it will never understate your speed:. But it usually does, that’s the whole point. At 60mph on the satnav, the Speedo shows about 56, varying as you stated with tyre wear. So it does understate. What it won’t do is understate to the extent where you may get a fine as it’s margin for error is always on the safe side. Surely?

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