1985 Supra speedometer 20MPH over what it should be

[chrismc410]

Hi

I bought a 1985 Supra recently and discovered that the speedometer is 20MPH over what it should be. For example, if it says 40MPH, you're doing 20MPH and 85MPH, you're doing 65MPH. Its a manual transmission, as far as I know, the orginal, I've tried a new gauge cluster, same thing. I"m told I might need a gear to slow the cable down, but I have no idea what cable I need or how to find it. I'm told there are five possible gears. As far as I know, the car is stock, engine, wheels match exactly what the glove box says, so what can be done and who can do this kind of work?

Thanks