in a parking lot make a paint mark in the middle of the tread at ~8 O'clock then roll the car forward straight ahead (~8 ft) till it leaves another mark & tape measure that distance (rollout) then divide Pi (3.14) into that number to get the exact dia then go to the archives chart along with your gear ratio (locate the chart that goes down to the tenths of an inch IE 26.2. jack up the car way high on the dr side to minimize fluid loss & be sure to clock the adapter on the side of the trans correctly for the number of teeth pinion you will be using.