Tire/Spedometer?

Ok, I have a 74 dodge dart swinger, I have a dash out of a 4 door, it only goes up to 100, im not sure if the gears in this is different than the ones from the 2 door that went up to 120 so the mph would be off...But i am riding on 15" wheels...Im wondering which is the problem and if it is the tires, im thinking it is 5mph off until i hit about 60 then its about 10 or more off...If someone knows the amount of change there is I would love to know, I never found a speedtrap to really check.