I’ve long been aware of one chain of gas stations that openly charges cash customers less per gallon than credit card customers. 

Apparently it’s not alone.  According to a local paper, the Contra Costa Times:

You’re getting charged for using a credit card - up to 3 percent per gallon - and may not have known it.  …

Typically, those who pay with credit or debit cards are charged a fee that’s included in the advertised gas prices, and those who pay cash aren’t charged that fee - between 1.5 percent and 3 percent per gallon.

Who knew?

Yet another reason to use a gas rewards card that pays 3% to 5% for gas purchases.  Otherwise you’re subsidizing everybody else that does, not to mention paying too much for gas.