Dear Tom and Ray:

I've had this 2001 Chevy Cavalier for almost five years. It's been a fantastic car. I take care of it, and it takes care of me. Here's the problem: Every year when the temperature starts to drop, my speedometer goes haywire. The outside temp only has to drop into the 40s for this to start. Morning, afternoon, evening -- it doesn't matter. One minute I'm right on the speed limit, the next it's saying I'm doing 110 mph. I can be at a stoplight, and the speedometer still will read somewhere between 20 and 100 mph or higher. Doing the math on the interstate and side roads to calculate my true speed is really starting to irritate me. What could be causing this, and how can I fix it?

-- Mary

TOM: You can fix it with a new instrument cluster, Mary.

RAY: This particular instrument cluster uses little stepper motors for the speedometer and tachometer. And because those two gauges get the most use, they tend to fail first.

TOM: There's no speedometer cable in this car. Information about the car's speed is sent to the speedometer by the vehicle speed sensor, or VSS. But it's very unlikely that a bad VSS would cause these particular symptoms.

RAY: I'm guessing you're going to have to get your instrument cluster repaired or replaced.

TOM: Getting a new one from your Chevy dealer will cost you hundreds of dollars. But you don't need to buy a new one -- after all, the rest of the car is more than a decade old.

RAY: I'd look for a mechanic who's willing to source one for you from a junkyard. Of course, the danger here is that you'll get a used one with the same problem, since it's a fairly common complaint.

TOM: But your mechanic can at least get a warranty on the part so that if it turns out to be faulty, he can go back and get you another one. You'd still owe him for some additional labor, but not too much, since he'd be able to test the part before putting the car back together.

RAY: And given the age of the car, a used instrument cluster will take this car either into deep old age or until something much more serious breaks. Good luck, Mary.