Science Questions

Why don't my sat nav and speedometer agree?

Sun, 18th Nov 2012

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Joanne Sexton asked:



Why does my car sat nav say I am travelling at a different speed to my car speedometer. Usually the speedometer says I am going faster than the Sat Nav so which is more accurate?



Joanne Sexton





Dominic -   Itís partly a safety measure that car speedometers tend to read a little bit on the high side.  That means if there's any slight uncertainty in exactly how accurate that speedometer is, if you're driving along in a 40 mile/hour speed limit balancing your needle on 40 miles an hour, at least youíve got a pretty good idea that you're not speeding.  You might be doing a bit under 40, but you're not actually breaking the law.  There's also some uncertainty because the car doesnít know exactly how big the wheel of the car is.  It only knows how fast that wheel is rotating.  If your wheel is slightly bigger then itís got a bigger circumference to that wheel and you'll be traveling further for each rotation of that wheel.

Chris -   In other words, when youíve put in new tyres on the car for example, then the wheel is actually going to go further for each rotation than when the tires have worn down, and for that reason, the speedometer doesnít know whether the tyres are new or old.

Dominic -   Yes, so to err on the safe side, it will assume that youíve got the biggest possible tyre that you might have and maybe even slightly bigger than that by 5 to 10%.  Whereas the satnav system, it knows exactly where you are on the surface of the Earth.  It can work out exactly how fast you're traveling and so, that will tell you exactly the right speed.


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Errors can creep into the satnav readings if the terrain is hilly so if you wish to calibrate your speedometer from the satnav readings choose level ground. syhprum, Sun, 25th Nov 2012

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