Weird, but I have a 91 F-150 w/302. When the outside temperature drops below freezing (into the 20's) the speedometer is about 15-20 MPH "fast". I have to guess my speed by the RPM guage. As the inside of the cab warms up, it seems that the speedometer error gets less and less. During the rest of the year the speedometer is fine. There are no problems with my cruise control, although I don't use it in the winter (snow). Is it that my speedometer is bad and needs replaced or something else?

Any help would be appreciated.

 

Mine is worse than that. At about 35mph it will read 75. I never bother to look at it because it only happens when cold below the teens.