Smithers Scientific Services conducted a whole bunch of tire tests for the califonia Energy Commission. I summarize the presentation here:
Barry's Tire Tech
About half way down, I discuss the size study and develop a formula based on the data.
RRC = 0.00246493 - 0.00000208*(Section Width in mm) - 0.00000386*(Aspect Ratio in whole numbers) - 0.00004700*(Rim Diameter in inches)
Short version: Bigger is better.