I have a question about something I read in another thread. Is it harmful to overfill your gas tank, like all the way up into the neck? I have done this quite a bit because I felt like it was a good way to accurately check gas mileage, by squeezing in as much gas as possible on consecutive fill-ups to measure exactly how much gas was used per miles driven. Someone stated that it could damage your evaporative canister, which had never occurred to me. If this could be harmful, I won't do it any more. What do you think?