I have logged 1500 miles on my Gen 3 Prius 2, and averaged 55 mpg on every tank, strictly used in my 70 mile per day highway commute. same speeds (65 mph) highway, 4 miles on and off interstate, same speed, no traffic. My stock tires (Yokohama S33D Avid) have been inflated to 37.5 psi, from the dealer new. Monday, I inflated all tires to 42 psi, and every day since my daily and tank average is 50mpg - no other change in commute, weather has been consistent. The change was immediate and very real. Going back to 37.5 next week for confirmation. Just a surprised FYI.