It's actually not cheaper because they tack on $10 or $20 or whatever the charge is for the car wash to the bottom of your bill after charging you the "lower" price for gas. Usually they're good for a week or two and they're hoping you'll forget about it altogether but even if you do use the car wash, what they have charged you is 1000% more than the car wash costs them. It's scammy all the way around, very typically American, sadly.