How accurate is your speedometer
 

I went to visit my sister this weekend and I was curious about how off my speedometer was. Those 'what's your speed' signs on the side of the road had been showing that my car was right on the money and so I counted how far off the speedometer was by watching the mile markers for 100 miles.

Over 100 miles of road the car got ahead by 2 tenths of a mile. So it is only .2% fast which I didn't really think any cars did except police speedos. My old car was 10% slow so when I was doing 77 it was reading 70.

Marvin used to confuse the hell out of those speed signs for some reason, coming towards one it would be jumping up and down by 10mph each way. However on some measured miles posted on one piece of road, he seemed fairly accurate, but there's only 2 miles worth.

Bear in mind that you're only within .2% until you wear your tires some or get new ones. From full tread to barely legal is a 2-3% difference.

I was comparing my wife's speedometer to the speed indication on her GPS.

Originally, the speedometer was about 1 MPH off from the GPS (highway speeds). Then I started to wonder if her air pressure was high engouh, and after I cranked the air pressure from ~30 to ~36, the speedometer matched the GPS exactly.

-Bob C.

Looking at the spare that's never been used and comparing it to the tires on the car it seems they are just a tad over halfway worn. they still have a lot of tread on them tho.

Mine's about 2% fast with new tires, to 4% fast with old worn tires.

mines is 5 mph over what i should be, if im going 15, im actually going 10 mph. the scanguage gives me an accutate speed cause i called a friend while were both on the highway, the scangauge matched what he said on his speedometer

Does it read 5mph over at every speed, or at highway speed? Speedometer inaccuracy is usually a percentage, so a speedometer that's 10% off would read 5mph off at 50 but 1mph off at 10 (and 8mph off at 80).

If it reads 5mph over at all times, it should be easy to fix by re-aligning the needle or the motor that moves it.

At highway speed my speedometer reading is within 1 MPH of my GPS. Usually it reads 1 MPH slower than my speed reported by the GPS.

-Jay

I thought all speedometers were supposed to error on the fast side by a certain margin?

On the highway I never really look at the speedometer. I look at my GPS for my speed.

-Jay