Edit (06/22/18, 9:10 am): the owner contacted me to say they limit discounts to one per customer.
I completely understand that. However, the 0.20 per gallon off was 2.34, while the car wash was discounted by 12.50. If I were given the option at the pump, I would choose the car wash discount every time.
I was not trying to game the system to get both, since the manager refunded me my wash minus the 2.34. I was more than happy to pay my fair price; the fair price on Wednesday is different than any other day and should be properly advertised as such.