My 2004 still had the original 12v battery which was getting weak, and I was putting off its replacement thinking it wasn't so bad. I was getting around 51 MPG with careful driving and mods. I installed a new battery and mileage immediately averaged 56 MPG. Although it may coincide with warmer weather (45-57F) and (possibly?) from a different blend of fuel, it seems significant. My current tank is averaging 62 MPG with a little more careful driving effort, which is way beyond what I ever got before, so I really think the fresh 12v battery is making a major difference. Just thought I would put that out there for anyone thinking of replacing their 12v battery, but putting if off for the future.
