Speedometer question

[HoustonLX]

Who here has found their speedomoter to be way off? A few months back I paced my car behind my wifes truck and found my speedometer showed 3-4 mph faster than she said we were going at 70 mph. Then a few weeks back I took a long highway trip and took a navigation system with me. That navigation system can show you how fast you are traveling and it too showed me to be going slower then shown. I also passed one of the mph signs police like to put out to show you exactly what your speed is. What I found is to do an honest 75 mph, I have to show to be doing 79-80 mph. I can understand these things being off 1-2 mph, but 5 mph at 75 mph? Any reason for this and is there a fix? My car is factory stock for the most part and run factory size tires (215/50/16).

 

[hetfield]

My SVT used to be about 4-5mph off. I fixed it when I put different needles in by using an OBD-II scanner to give me the vehicle speed directly from the computer and placing the needle carefully. I happened to nail it on the first try

 

[Dyoel182]

I need to go do that with my GPS unit sometime soon.

 

[95pgt-t]

its how ford gave svt owners the impression that they were going faster than they were. makes you think the car is quicker than it is:laugh: