It's not really a MPG enhancer but it increases your $MPG! They seem to be an easy way to get 5% more mileage for free. Currently I am using the premier cash back card with Bank of America to get 4.5% cashback on gas up to $2,500 per quarter all year. If you don't qualify for premier you only get 3% back.
BOA Cash Rewards Card
Other options include rotating categories such as Chase card or Discover card to get 5% during one quarter per year.
Some lesser known ones are Fort Knox Federal Credit Union Visa Platinum unlimited 5% back gas card.
Also Ducks unlimited cash back 5% Gas card
I might try one of the funky ones eventually but for now the BOA one has been working for me. What card do you guys use or do you try to only pay cash?
We have a chain in NJ called Diesel and Gas (At least they have the same paint job) that only takes cash but they are the lowest advertised prices. I never use them because the 4.5% back ends up being cheaper elsewhere. My "go to" station has a $0.10 discount every Wednesday I try to hit. I am pretty consistently $0.20-$0.40 bellow the AAA national average even in NJ with Full Service only.