Speedo or GPS? Which one is right?

[MartyE]

Lads and lasses. Here's one for you.

I have had Tom Tom 3 since Christmas and been on the motorways quite a few times since then, both in my car which is a year old, and my wife's car which is three years old.

The speed on the GPS tells me I am doing about 5mph to 7mph less than the speedo in my car and similarly in my wife's car.

Is the GPS more accurate than the speedo on the car dash - even though the signal is travelling into space and bouncing off a sat? Or do you think the car is more likely to be right.

If it turns out to be the GPS then that means we can all probably drive that little bit faster and still not be speeding because all of our cars will be a few MPH out.

Don't blame me if you get a ticket though.

Puzzled of Sunlun by the Sea.

[chris_bramall]

I think by law all speedos must read either at or below the actual speed by a percentage.

this is to account for errors due to tyre pressure, tyre wear and general errors in an uncalibrated system. The only way to find which one is right is to get clocked on a calibrated speed measurement device and find out what the speed says.

[nobby.nobbs]

both have % errors. speedo's are inaccurate due to tyre pressures and 1000 other minor glitches in any mass produced product. gps speed only as good as your gps unit and the satellite reception. in good reception with good gps then it will have the most accurate reading.
howver, the police car with the calibrated camera,vasco system will win the argument in court everyday. you pays your money......