Actually, I think I just figured out why the lower tire warranty. The same thing that makes high profile tires more efficient is also the reason why low profile tires have reduced longevity. Better flotation and wider sidewalls make such tires conform to irregular road surfaces better. Less squirming, hopping, and rubbing means lower rolling resistance. The opposite of this occurs on low profile tires, increasing rolling resistance and... wear! Thus manufacturers like Michelin rate the 16" tires for 65,000 miles, and the 18" tires for 55,000 miles. Mind you, they also come from different model lines which might well make a difference too.