Banks4004

My speedo has been off for about 4 years since I did my 308 to 342 diff swap, last time the trans was out I could not find the right gear anywhere so its still off. I recently did the tach fix and it was so easy, just soldered in a pot to the tach calibration resistor. Does anyone know if there is something similar that controls the speedo calibration and if something as simple as varying the resistance could correct it?

Thanks