The taller tire will have more sidewall which will better absorb defects in the road.
To the degree that the tire deflects, in theory, this would decrease mileage over a very thin hard rubber tire. (Take a look at the wheels and tires on high mpg specialty cars - they are almost like a hard rubber bicycle tire)

However, a thin tire - much like the jokers who put 25" wheels with low profile tires on SUVs, is going to give you a bottom jarring ride.

In theory the thin tire will give better mileage, however the question is if this would translate to any real practical gain in a real wold situation. Personally, I'd go for the tall sidewall - particulalry if I were going to drive on anything other than smooth freeway.

Todd
'88 Westy

-------------- Original message --------------
From: Joel Sell <justthesells@YAHOO.COM>

> I am interested to see where the "under the front end aerodynamics" thread goes,
> but I'm also wondering about wheels and tires. Short of purchasing racing wheels
> which I think are illegal to use on the street, what are our options for
> lightening up our wheels? Also, Is a 14" wheel with a tall tire better than an
> 18" wheel with a low profile tire? I know that thinner is better, and it seems
> that as wheels increase in diameter, they also increase in width ,but assuming
> you could stay thin, is a tire with a tall sidewall better/worse than a tire
> with a low sidewall? How about overall diameter?
> August '06 Popular Mechanics says that "rotating weight of a spinning tire is
> 1.5 times its actual mass- so this is an area where trimming pounds really pays
> off".
> Any thoughts on this?
> Joel in Philly