I drove by a few of those speed detectors that they put on the side of the road that tell you how fast your going and my speedo is off by 3mph each time I go passed one of those things. The speedo says i'm going faster than I actually am. Why would this be. I didn't change tire sizes of rims or anything. Its not putting extra mileage on cuz I compared the actual miles to my work both on my truck and my mustang and they are both the same and the mustang speedo is dead on the actual mph. Any ideas?

 

That's normal.

I'm not sure when or why the car makers started doing this. But I've noticed it on many newer cars. My 2006 ranger and my 2001 caddy both read 2mph faster at 70mph. (measured against both GPS and those road side police radar signs)

Interestingly.. the PCM will show the speed dead on with GPS and those signs. But the speedo's don't.

Rich

 

Take off the needles and fix it yourself. Thats what I did.