out in the parking lot put a rear valve stem at 6 o'clock then roll the car forward in a straight line till the stem returns to the same position (grab several bricks & your tape) & measure "rollout" then divide Pi into that number for your diameter then as said go to the chart & CNXT (SP) priorly posted a comprehensive chart that is in the tenths on diameter numbers & your gear ratio. Jack the vehicle way up on the DR side to minimize fluid loss.