Dang, how fast(or slow) were you driving to get 34mpg? Thats 10 more than I can manage, but I already know my car has unusually bad economy. The plugs are new Autolite 764s, but I think the cap, rotor, and wires are original plus the engine harness is falling apart.
All my highway driving is at 70+ so its taching up at 2900-3100rpm.
I was curious, so I did the calculations. Assuming you have a 12gal tank, buy $3.799 premium, and get 31mpg, it costs $0.12/mile. With $3.499 regular and 25mpg, it costs $0.14 per mile. I guess it's kinda like paying more for diesel to get the extra economy.