Not aware of any speedometer that reads from "A" tire, or even from the rear end itself, most are driven from the tailshaft of the trans. So we know the differential can deal with two tires of different size, there is still only one driveshaft turning at x speed. So as Jim said in first post so 2 tires turning at different speeds and one driveline turning creates the speedo reading. As is, 1 tall and 1 short tire, speedo reads lets pretend 30 mph 2 short tires and speedo reads 35 mph 2 tall tires and speedo reads 25 mph As it is, it's simply broken, and needs repaired with 2 tires of the same size. Then op can figure out what height tires he needs to get correct speedo reading. Maybe he can borrow some from another car, before they spend hundreds of dollars on new tires?