Michigan: Do you know about getting a discount at the gas pump when you pay with cash?

Susan Tompor writes in today's Detroit Free Press about the issue of getting a cash discount at a gas station when you use a debit card which is the same as cash.  She writes like it's common knowledge that everybody knows that when you throw down cash for gas, rather than a credit card, you get a discount.

I didn't know that.  I do stay abreast of most things that cost me or save me money.

Anybody familiar with this?  How much is the discount?  Is this something that every station does?