The mighty penny (Update: RIP the penny)

How bout that wacky gas station pricing? $3.09 & 9/10ths cent per gallon? I don't think I have ever have seen it explained to me. If I go and purchase one gallon at the above listed price, I certainly cannot use legal US tender to give the cashier $3.09 and 9/10ths of a cent, or get my 1/10th of a cent change back. Just round it up already.