Refering to my assertion of yesterday, I, indeed, didn't know what I was talking
about :).  I was wrong when I said, "This effect is more than an additive one."
In actual fact the increased effect is only additive and not "by the square",
exponential, logarithimic, or such.  As the radius of the wheel/tire changes,
you are simply changing the length of the effective lever arm with the axle
center its fulcrum.

-Dana-

I may or may not know what I'm talking about, but...one other consideration when
deciding on wheel/tire combos is that as you increase the diameter/width of the
wheel/tire you also increase the flywheel mass and the rotational inertia of the
wheel/tire at speed.  The added mass is concentrated at or near the
circumference, and will affect your Vanagon's acceleration, how you climb mt.
passes, how your brakes wear, and how you stop (you will be happy you installed
the larger brakes to complement the larger wheels).  This effect is more than an
additive one.  I believe increasing the wheel/tire diameter by 10% will give a
greater than 10% increase in the resultant inertia of the rotational mass.
Therefore increased effort/time of more than 10% to bring your Vanagon to a
halt.  Unless I'm wrong or half right :).

-Dana-