road angel /sat nav/speedometer

[regulators]

in my car i have a road angel and a sat nav that tells you your speed why is it that these two devices read the same as each other but theyread about 2 to 3 miles an hour slower than my cars speedo and which device is correct and most accurate

[AllYourBase]

The sat nav will be more accurate due to the way it works. It will calcuate your speed relative to the signals it recieves from the satelites it receieves data from. Using trigonometry and mathermatics the speed calculated will be very accurate.

Speedos only give a decent idea of your speed, but can be affected by variations in wheel and tyre size etc.

[qapmoc]

The average speedo is a low cost instrument, don't expect better than a 5% error on the true->indicated speed, that is assuming you have the correct size tyres fitted and have not changed to low profile or something and that the tyre pressures are correct.

[Trotbot]

Speedos tend to be set to show a slightly faster speed than the true speed. Manufacturers don't want to be sued by speeding motorists claiming it is the manufacturers fault

[pagey]

Speedos are permitted to slightly overestimate but never underestimate the true road speed. If you are enything like me you may find yourself using the speedos on the satnav etc to dictate how fast you drive and not the car speedo - but beware that satnav speedos can UNDERESTIMATE your speed but rarely overestimate - so they can say you are going slower than you actually are when gps signals become partially lost.