We have a 2004 Chevy Cavalier. When it is hot out, the speedometer is very erratic. Either saying you are doing 90 when you are doing 40 or saying you are going very slow when you are not. It doesn’t seem to be doing this when it is cool out. It also doesn’t do this all the time. It may go days without acting up. We changed the variable speed sensor on the transmission (it was only a $13 part) verses the $500 instrument panel the dealer told me was the problem without even looking at it! I just wonder why it would act this way when it is hot but not when it is cool. The car has less than 40,000 miles on it but I think just passed the 36,000 mile warranty! Figures!
If the problem was an erratic VSS, you would probably have gotten a trouble code, and the CEL light would be on. I’m afraid it is most likely the speedo cluster. Try f a salvage yard for the cluster at a lot less. This is a part I wouldn’t mind buying used.
Is that the whole instrument cluster panel with the tach and gas gauge and all that or is it just a part behind the panel itself that hooks into the instrument panel (like a cable and connection port or something similar)?