My highway milage seems lower than normal. It has since last fall, since right around the time my car also ran its slowest 1/4 times since I put in my gears. So, I just finally put 2 and 2 together and decided taht maybe its the plugs although I haven't checked them yet. They've only been in there for about 20k miles though. What is anyone else experience with the 12C plugs? Are they normally pretty short lived?