I would measure rollout in a parking lot, the distance starting with a rear valve stem at 6 O'clock then rolling it straight forward till the stem is back at 6 O'clock (~8ft) then divide Pi into that number for the exact diameter. With that number & your gear ratio then pick the correct pinion teeth number from the chart CNXT posted. this will get your ODO correct & if the speedo magnetic gismo is working correct, the speedo will also be dead on (you did say it was rebuilt). what you could do right now is either use GPS if you have it or go 5 mile markers on the interstate & see if the ODO has the same error % that the speedo does & if so then with the correct tooth count swapped in the trans you should be good.