Why don't my sat nav and speedometer agree?

Dominic - It's 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 you've 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 doesn't 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 it's got a bigger circumference to that wheel and you'll be traveling further for each rotation of that wheel.

Chris - In other words, when you've put in new tyres on the car, for example, then the wheel is actually going to go further for each rotation than when the tyres have worn down, and for that reason, the speedometer doesn't know whether the tyres are new or old.

Dominic - Yes, so to err on the safe side, it will assume that you've 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...