I tried that today and it shows I'm going faster than I actually am. The truck has bigger than factory tires so that's backwards isn't it?If you have any kind of GPS like a Garmin very accurately shows your speed.
Ha, use to go to an interstate and use a stop watch between mile post, driving at 60mph, should cover a mile in precisely 60 seconds. If say takes 65 seconds take the ratio of 60/65 times 60, speedometer 4.7 mph slower than 60.
With GPS just compare that number with your speedometer. These new cars are really quite accurate