Stanster...do the mustard test with the old tires and then the new tires.
Gets you an accurate measure of the true rolling radius.  Since the torque
applied through the moment arm of the rolling radius is the force at the
tire to ground interface you can make an easy calculation, even in Texas,
even without a cat, that will get the relative difference.

EG:  if the original rr is 13" and the new rr is 14" then the difference is
14/13 reduction in tractive force.

If you had 50lbs-ft at the axle you would have 50/((12/13)*(13/14))lbs of
force with the new size.

No cats were used in the garnering the data above, even in texas.

p