Even GPS has its issues. Spurious points off to the side of your track will cause GPS to think you traveled farther.

Interstate mile markers too. I've found examples where they deviate a surprising amount. It seems to happen when the two directions (say eastbound vs. westbound) veer away from each other or around obstacles. They "adjust" the mile markers so they line back up after the highway comes back together. Or when the route changes a bit with new construction, they don't runumber all the way to the state line, they adjust locally.

 

A method I have used to check accuracy of speedometer is to drive for 6 minutes (1/10 hr) at constant speed w/ speed control, beginning time at a milepost. Measure whole miles from mileposts and tenths of miles past last milepost w/ odometer. Simply multiply distance traveled by 10 to get mph over that 6 minute interval. This is reasonably accurate (< 5 %) even though odometer has same error as speedometer since you only use odometer for the tenths of miles.

I usually repeat 2-3 times to get assurance of result. Easy to do while driving.

good luck

 

Hmmm, maybe not all GPS units are created equal; I've had three and every one of them was accurate to well beyond one one-hundredth of a mile.

 

Some GPS units show current speed too, not just mileage/distance traveled.

 

Yup, all of mine had/have kind of a "Dashboard View" mode.