It's also a good idea to watch credit card statements. I was making a cc payment online last week and on my recent list of charges noticed that from one specific gas station where I'd topped of with my cc, I'd been charged two separate $1 fees in addition to the price of the gas itself...apparently just for using my cc. When charges like that are piggybacked in at the pump, it sort of negates any discounts in the long run that a person may "think" they're getting.
ETA: I did make a recent interstate trip, but as I was towing a 6'x12' enclosed U-Haul trailer, don't think my recorded gas mileage averages will do anyone any good in this thread.