Most speedometers have tolerances of plus or minus 10%. However, I would say yours is too far off. You can take it to ford (or another shop) and they can reprogram it. I believe at ford it costs around $70. I had them reprogram mine when I got new tires, but this was 10 years ago. Or you can buy an SCT tuner which allows you to adjust it yourself. With a tune those run around $400. You do get the benefit of a new tune and the ability to change it in the future if you change tires/gears.
Mods-32 BFG K02s, SCT 91 Tune, Flowmaster Exhaust, 4.10 Ford Racing Gears, Tinted Windows, Tonneau Cover, Kenwood Headunit, Infinity Kappa Speakers, 12 in Alpine Type-R sub, 4x50w and 1x600w Alpine Amps, Signal Mirror Kit, Custom Painted Interior, HIDs, 2 150w KC 6" lights, Mustang Dome Light