I boosted psi to 40# and noticed a much better feel for the road (I don't like mushy) and a slight improvement in mpg. I don't have figures to hand -- I loaned out the HCH to a friend who's in PA (his Subaru is in the shop). My results are also skewed by my passengers' requirement for a/c. (The only reason I don't gripe about summers in DC is I've lived in Chicago and Houston -- and NYC one summer. This ain't bad.)

Is it really safe to run at max psi?

Another question, speaking of a/c: Does driving with the windows open really affect fuel efficiency, by changing the aerodynamics of the car?