You can easily get the rollout figure to a much better degree of accuracy
buy using the mustard method.  Make a line of good ol french's yaller
mustard in front of your rear wheel.  have the van loaded as you usually
travel.  drive straight through the mustard until you've done two wheel
rotations.  You can watch out the driver side to see.  Measure leading edge
to leading edge with a serious metal tape measure over the three marks.
This will give rollout times 2 accurate to ~1/4 inch.  Frank will use Grey
Poupon, BenT will borrow some generic mustard from the Jollibee but they'll
both get the reading.

Plug the rollout for one rotation in as the distance travelled per wheel
rotation. (duh).

Work the gear ratios backwards to determine motor revolutions per mile in
the various gears.  Plot the results.