Spoiled said:
but then you have tire width... and the pressure of the tires, flat ones will have more contact with the road... the flaw in that argument is even if the heaver vehicle applied less PSI its doing it to a larger area, which means its still adding more wear on the roads...
And I'm a structural engineer... so let me tell ya that's not quite true.
Every material has a yield stress, measured in PSI. Meaning the material can withstand up to the yield stress without damaging the material. Once you reach that yield stress, the material starts to sustain damage.
Hence, lower PSI's, no matter the area covered, means lower stress, and assuming that at all points where the tire meets the road is lower than the yield stress, no damage is occurring.
Now, my expertise ends at metals because I'm not a civil engineer, but I can tell you that asphalt, concrete, etc. *probably* have characteristics very similar to homogenous metals. The only difference are that they are more of a composite, which means they are stronger in some directions than other directions. (tension vs. compression, tension in 1 direction vs. tension in another direction, etc.)
Same goes for fatigue stress. Assuming the PSI is low enough, you won't experience as much wear, even if you spread the areas.
Now, there are phenomenons called fretting, bearing stress, etc., but that's probably too deep a discussion here. The wear that occurs on roads, if I had to venture an educated guess, is probably due to friction, grinding, heating and cooling of the road surface, and water.