Is my speedometer accurate?


You might feel like you’re being almost rear-ended or you might notice other vehicles frequently overtake you, but you never quite understood why.

Your vehicle’s odometer reads that you’re driving within the acceptable limit.

But wait!? It’s not until you get your sat nav out that you start to realise a discrepancy between readings on the car dashboard and the nav.

Margin of error in a car speedometer

Wheel diameter differences due to things like worn tyres, less air in the tyres or different brands used could be minuscule.

But at 30mph, with car wheels rotating 6 to 7 times per second, a difference of a few miles an hour is easy to rack up.

That’s why there is a margin for error taken into account in how the law is applied and how manufacturers calibrate their speedos.

Now what about the sat nav?

tom tom sat navSat navs, whether integrated or portable to the car, will calculate a car’s speed based on GPS satellite tracking.

These units continuously track your exact location via satellite and then calculate how far you’ve travelled divided by the time it took you to reach that distance. Yes, we all remember basic Science O Level.

The accuracy of a sat nav is ultimately determined by the quality of the satellite signals and not your vehicle’s tyres.

Vertical direction however is not taken into account in many sat navs, so moving up or down steep hills can yield an inaccurate speed.

Over larger distances, for example, those covered on the motorway, you will get the most accurate speed reading from your sat nav. Rounding errors are largely eliminated.



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