A local gas station here in Kentucky gives a 10cent/gallon discount for using debit instead of credit. Not sure if it's a permanent change, but last time I filled they had the pump programmed to offer it.
I'm not sure how much cheaper a debit card is for companies, but I've read it protects you less from fraud.
It doesn't have to go through the big card companies, so I'd imagine they're saving more than $.10/gallon. Nevertheless, I feel like I'm supporting business instead of banks when I use debit. (Also, myself. That's like $1.50 cheaper for no effort)