David and I paid more per gallon at a gas station in San Diego a few weeks ago than we intended because we paid the credit card price instead of the billboard advertised price which was for cash only. Gr.
Apparently the trick is being used around the country. In some cases the credit card price isn't even posted, the price just changes on the meter once you swipe your card.
If you encounter a cash-only price and don't have enough cash on you (who does these days?), ask the cashier if you can pay inside with your debit card.
Azcentral.com has an interesting twist on it:
Pay with cash, pay less.