Forgive me if this has been covered before I did a search and couldn't really find this problem.
my speedometer and my GPS have always been within 1 mile per hour of each other.... today I did a cruise and everything was varying by as much as six to eight miles an hour.... with my GPS saying I was going slower than my speedometer. When I pulled in my driveway I stopped and looked at the speedometer and it was showing I was going 6 miles an hour at a dead stop. I just had the car down to the Ford dealership for an oil change but can't imagine anything that would have been touched or bumped that could have caused this.... any ideas do I have some kind of sensor going bad maybe?

 

I wish I could help you, but I have no idea how these new fangled speedometers works. Back in the day you could change the cable or gears in the tranny, and all would be fine. Hopefully giving you this "bump" will help get someone that knows to chime in.

 

Well.... thank you for the bump but the crazy thing is the last two trips I took it worked flawlessly so at this point it's not even testable!

 

I didn't reply earlier because I didn't have a good answer but I guess something is better than nothing . . .

Usually if the speedometer is mis-calibrated, it is off by a percentage; and zero is still zero. It is very strange for it to be reading any value when the car is stopped. Maybe it was just stuck somehow.

Calibrating on these cars is actually easier than the old fashioned mechanical speedometers, if you have a "tuner" aka programmer, you just plug in the required numbers (rear end gear ratio and rear tire revolutions/mile) and the computer does the math.