Pre-Paid Gas

So the other day I had to stop to get gas and all I had was cash. I normally pay for my gas at the pump with my bank card so I got to experience the whole pre-pay process. I think I've only had to do that once or twice before. I know it hasn't happened very often. My question is, how does someone who has to pre-pay ever get there gas tank completely full? I wouldn't think you would be able to guess exactly how much it would cost to fill your tank? I know I couldn't. If you pre-pay too little then you won't get a full tank. If you pre-pay too much then how does that work? If you don't fit it all in your gas tank do you go back inside and get the difference back? Anyone know? Someone enlighten me on all this as I am pretty clueless obviously but definitely curious as to how it all works!