Credit Cards For Fuel Discounts

I use my Costco credit card at Costco gas stations and receive 4% cash back on fuel. Recently fueled up with diesel at $6.00 per gallon, netted out at $5.76. They have the lowest price locally plus cash back. This discount works at dedicated gas stations, but not at stores that have stations like Safeway and Walmart.

Downside is, even though there are plenty of Costco stations, very few have diesel. I have an auxiliary tank so I can manage short trips up and back without refueling. This has worked well for me and my travelling style.