And now for something to really mess with your head.... tires companies
do no provide precise size measurements.

Try a number, use it for a while, it will seem high or low. So change it a bit and
see if you can get closer. You can also try to measure by seeing how far the tire actually rolls. I found that more trouble than it was worth. I just want to be close, and if the mileage reads a little on the generous side, I am not one to complain.