Re: 2004R Speedometer Calibration
If you plan on keeping the 255/60-15's, or a tire with the same overall diameter which is approx 27", you need a 10 tooth drive gear which is green for the 200-4R and original to the Monte Carlo SS's, and a 28 tooth driven(yellow).
You could get by with an 11 tooth drive gear (orange) and a 31 tooth driven (white).
I would pull your existing driven gear and look in the hole with a flashlight and see what you have for a drive gear.
GM P/N's are 416765 is the 28 tooth driven about 5 bucks at the dealer.
8639906 on the 10 tooth drive gear, lists for $42.