About your mileage, I have a story. I had a Toyota Windom (Lexus ES300) which was rated for about 22 mpg us and I averaged about 19. The mass air flow sensor was bad, so I bought a cheap auto part store version to replace it. From that day on, the car never dipped below 25 mpg and I even managed 39 on the highway. Just out of curiosity I pulled plugs and discovered that it was running on the lean side (quite lean) but still within safe limits. So, in short, you may already have a lean burn Corolla unknowingly.
