I have a 2003 Chevrolet Silverado 1/2 ton pickup. Since it started getting cold the speedometer has been giving me problems. When I first start out in the mornings, it doesn’t register my speed. Within a mile or so it will start to climb, it’s usually up to a 10 mph at this point. The further I go the closer it gets to the right speed. After five or ten miles it seems to be reading the correct speed, and I don’t have any more problems unless it’s still cold, and the truck sits for a while. Where would the problem be, in the speedometer itself, or elsewhere?
Stepper motors on the instrument cluster are going out. GM did a goodwill warranty on these up to, say, 75000 miles. If you’re just out of that, perhaps a dealer will work with you. Wouldn’t hurt to ask.
The alternative is to replace the cluster. It’s an easy do-it-yourself, and there are plenty of online places (check ebay) that will send you a new cluster in exchange for your old one. Expect to pay to have the mileage programmed into the new one–around $100 or an hour of labor.