insidious_machinae

I know this answer for a fact: Arco gas is cheap for exactly the reason Hambone and Matt_Freeman said, just like Costco's gas. They don't let you use credit cards to pay for it, generally speaking, so every time you would normally cause them to pay for the credit transaction, they save some money. This money obviously adds up, so they give a small percentage of that back to the consumers by setting the gas price lower.