Just bought an 01 GT and the speedometer reads about 25 MPH faster than I am actually traveling. I'll have to check to see if I have large tires than stock and if I do though its not by much.... I don't know what else could be the cause of it though.
Typically if it reads that drastic of a difference, that means someone has changed the rear end gear ratio. I would determine if that has been done, and if so, you would need part number LRS-SPDCAL2 if it is a manual transmission car to correct it.