JohnF, as an addition to my post above, watch this video at about the
1:00 minute point.

http://www.youtube.com/watch?v=EEuAqq8FINw

You will see that it's moving very slowly (early in the self start
process) and if you note the streamers and the angle of the prop
airfoils it's not hard to see that the relative tailwind hitting the
blades wants to turn the rotor CCW (from the rear) while the wheels
are in fact slowly driving the rotor CW.

It is this battle between the direct forces of the wind on the blades
and the force of the wheels driving the prop shaft that is influenced
by the gearing ratio you choose (or wheels size .. same thing). If
you lower the gear ratio, the blades win, the rotor turns CCW and the
vehicle backs into the wind. In the configuration in the video, the
gearing is such that the wheels win the force battle and it moves
downwind.

The closer you move to this 'center point' in gearing (without going
over), the faster the vehicle can go downwind, but the longer it takes
to get going.

JB