I have card a AMEX, from Costco and snce it is a commerical account I get 3% back on all purchases that I make at gas stations including Costco. You don't get the discounts at gas stations at places like Safeway. I also have a card with Flying J/ Petro that ives me a discount. I keep one of their booklets in my truck, and plan my gas stops around their locations. Their prices are normally the best, in the area, and you get a discount on dumping sewage with the card, if you need to dump. Costco always seem to have the best prices in an area.
I also sometime fill up even when I don't really need gas, to get the best prices. An example is Costco in Phoenix charges about 10 cent more per gallon than Prescott,AZ.
Have a geat trip this summer