another method of measuring the rolling circumference of a tire is to
mark the tire with a piece of masking tape at some point on the
sidewall at the bottom of the tire (where the tread is touching the
ground). now, just beside this mark/tape, make a mark on the ground.
roll the bus forward and count the revolutions of the tire as the
masking tape rolls around to the bottom again. let it go ten
revolutions and stop. mark the ground next to the tape again.

now measure between the two marks on the ground and divide by ten.
that's your rolling circumference.

it works best in a big empty parking lot, like a mall after closing on
Sunday or some such. and that way you can mark the 'ground' with the
masking tape, also.

joel