The discussion is really about torque and thrust.  Your engine has a torque
curve that relates torque to RPM.  This torque is multiplied by the gear
train (transmission gear x ring and pinion ratio) and appears as a twisting
force or torque at the axle.  This torque is applied to the road through a
moment arm which is the actual distance from the center of the axle to the
ground.  For the sake of example call this the rolling radius.  For any
given engine transmission differential instance the difference in torque
with various wheels can be calculated as a percentage.  This percentage is
the difference between the rolling radii of the two wheels in question.

EG:  Wheel/tire 'A' has a 13inch rolling radius.  Wheel/tire 'B' has a
15inch rolling radius.  The reduction in thrust by the larger 'B' wheel over
the smaller wheel 'A' is the ratio of the two radii or 13/15.

FWIW this process also works for retarding force or braking.  A larger wheel
will lessen the braking force per unit pressure by a similar ratio.

QED or something like that, rigorous proof is left as an excercise for those
with too much time on their hands.

A useful tool in the form of an MSExcel spreadsheet is available for posting
to the Vanagon Yahoo group.  Tim Smith is the original author.

P