Easiest way is not cheap. That would be to figure out how much in %, and then get a 'speedo adapter" gearbox. Takes less than a minute to install.
More work, but cheaper is to get the correct "driven" speedo gear. If you get lucky, you may not need to change the drive gear which is more work. You won't know until you find out the exact error you have in %, and then find out what drive & driven gears you have...then go to GM and they can look up the corrections. They may prefer you give them your axle ratio, tire size, etc, instead..
Way too much screwing around for me. Just ante up, and buy the adapter
By the way, you can't just say my speedo is off by 10. Get on a hiway with mile markers and compute the percentage.