Cheapest is costco, I know where they all are...plus I get a 3% rebate on fuel when using AMEX. That's ten cents a gallon.
I have a AAA Visa that gives me 5% off all my gas. Costco won't take credit. So it's a wash for me driving out of the way for the cheaper gas then NOT getting the rebate. Closest one is 8 miles round trip on the streets. That's a half a gallon of gas in my 2003 Astro loaded with tools.
So I can't really win there.