Although tire size and gear ratio play a part in what the speedometer reads, it is not necessarly the information you really need to correct/calibrate the speedometer.

The two things you need are:

the NUMBER of teeth on the speedometer driven gear (the color really doesn't help that much as I have seen the same color gear with different tooth counts).

The MPH the speedometer is off.......but that in and of itself is not enough. On the old analog speedometers the innacuracy will be off by a percentage, in other words if the speedometer is off by 5 MPH when driving 30 MPH it will be off by 10 MPH when driving 60 MPH. You need to provided the actual readings. I.E. at a vehicle speed of 70 MPH the speeedometer reads 62 MPH.

If you provie the 2 pieces of information I can figure out what speedometer gear you need.