Use the French method.  Using French's or some other less well calibrated
mustard, place a substantial line of mustard in front of the rear wheel.
Drive through the mustard until you have between two and three revolutions
of the front wheels.   Using a steel tape measure, a laser interferometer or
other suitable measuring device measure from the leading edge of the first
mustard mark to the leading edge of the third mustard mark (two rotations).
Accuracy to plus or minus one nanometer is not necessary.

This will give tire rotation times 2 in your favorite unit of measure with
enough accuracy to do practical calculations.

NB:  Drive in a straight line when marking, preferably after warming up the
normally inflated tires on your normally weighted vanagon.

With the rollout thus determined you can use the T. Smith tool to do all
sorts of calculations.

A copy of the MS Excel gearing tool can be found on the alternate list
(Yahoo Group Vanagon) or the Subaruvanagon group.

Cheers,

pensioner