If the odometer is correct, you probably do have the correct speedometer installed. Is it a 125 mph? For some reason the (replaced with 125 mph) speedo in my 82 suddenly did the same think a couple of years ago, the odometer stayed correct but the speed started reading significantly high. I disassembled it and did not see any obvious problems. I simply repositioned the needle (trial and error) until it was accurate (within 1-2 mph) at 80 mph using a stop watch and mile markers. It's been fine for about 2 years now, but I still don't know what happened. I know I didn't do a proper calibration, but I wasn't sure it would work at all. If you get to the bottom of this, please let me know.