I am not sure why the pumps default to credit when you put a debit card in. It may have to do with the fact that a debit card transaction impounds the maximum amount the pump will let you have each time you put the card in. That can impact people who carry a marginal bank balance. It may be more than you have. There is some debate about how long those funds are impounded before they finally bill out the actual amount. I can't think of a hardware/software reason why it isn't instantly when you hang up the nozzle but I understand it is not.
