We have to know what to replace it with. Check out "Speedo World". They have the gear you need for around $10. They will need to know: tire dia, rear axle ratio. With any luck that is all you will need to purchase (driven gear). Otherwise, you have to change out the "drive gear" also. More labor because you have to remove the driveshaft & tail-housing.
Does it say you are doing 25 when you are at a dead stop?
Go online and search GM speedometer gears. TCI has a good site. You can DIY or take it to a transmission shop for repair.
Have you changed the rear outer wheel diameter or the differential gears? Here's your problem, to change the speedometer gear ratio you have to pull the transmission tail shaft housing off to slip the new plastic speedometer drive gear on the tail shaft and change the cable drive gear. The problem usually lies in the viscus coupling on the speedometer magnetic speed cup and it's far easier to swap the dash gauge out to fix it right. The speedometer will only get worse over time, and eventually it will peg out with any appreciable amount of vehicle movement. I'll bet the odometer still reads properly, this is how you know if it's the speedometer speed coupling.
http://www.autotraderclassics.com/images...
http://htsmall.rickscamaros.com/assets/r...
Does it have bigger tires ?? There's a small plastic gear on the tranny end of the speedometer cable, in the fitting where the cable hooks on to the driver's side of the tranny.
Sounds like 3.42 gears will solve your problem.
