Your speedometer is calibrated so that 1000 rpm of the cable reads 60mph.

From that, what determines how fast the cable turns? Three things: your speedometer gear ratio in the transmission tail housing, your rear axle gears, and your tire diameter. Which one of these did you change? Factory speedos are within 5%, usually better. Over the years, someone changes the tranny, the rear axle or most likely the tires and the speedo reads wrong.

You can go to tirerack.com and look up your tire diamter. On the door post or in a book, find your original tire diameter.....

The speedo can be adjusted by changing the plastic gear in the transmission tail housing......simple job and a cheap part to buy from the chev dealer.