You're saturn explination works wonders when people are running stock tire sizes, but in most cases no one is. Going to a 4.36 can net you improved gas milage.

Comparing the amount of gas used between my car (4.08 & 255-0-17) and epsteins car (4.36 & 245-40-17) in july on our 2000+ mile deals gap trip we always put the same amount of gas in out cars. I checked how much gas all our cars where using at the time and we used up about the same amount every time.