All new cars are fitted with a +/-10% out tachometer, this means you are travelling at a speed of 60 in the dash, the actual road speed is in fact only 54-66, 80 = 72-88 or 100 = 91-110. As the tachometer can never be 100% all the time, the only sure way to know your true road speed is by GPS like you say.
I know for a fact my ute is out. I'm a full 10% off due to tacho being out(mine is out -5%) plus having a smaller circumference tyre than the recommended size tyre for the vehicle. So now If I want to do 60, I need to stay at a speed of 66. 100 speed limit I need to be at 110.
It's funny when people drive with you and you go past a fixed/mobile speed cameras at 88kms in an 80 zone, they don't see the "camera flash" and then go... How the hell did they miss that, 8kms more than the limit What The!
