I was thinking about switching to the Shell Mastercard which gives 5% on Shell only. Shell in my area (Chicago) is about average and rarely lower thasn other gases, although in some cases, 5% of Shell is a penny or so cheaper than 3% of something else, depending on which county you are in (Cook County is more expensive). I will buy any gas if cheap enough, so I am not a Shell "diehard." I will, however, buy Shell if the price is right. Shell advertises that their gas has been reformulated to give better mileage (than other gases?). Is this just advertising promo? Is the Shell Mastercard a better deal for me, or should I stick with the Chase Perfectcard, even at 3%?