I don't know where you're located but most major towns will have a speedometer shop - in the Detroit area it's Bob's Speedometers. They'll have some kind of chassis dyno so they can roll the car up and do a speed correction. Generally they'll make a small speed correction box to go between the trans and the speedo cable - problem fixed. At the worst they'll make you a correction table to post near the speedo but I've only seen that happen once at Bob's. BTW - when I worked at the GM Proving Grounds all of the test vehicles had that correction chart stuck to the speedo so us test drivers could drive at the actual speeds for the test cycle and not the indicated speed so they KNEW their speedos were off!

Dan