[poundwise]

A way you could not spend any in the week and still fill up more than once at the gas station is to buy a gift card at your local BP or Shell or whatever. In the same way some have said they would need to stock up on milk before beginning, you could stock up on gasoline.

You'd need to be wary though, it hardly counts as a week without spending if you just spend it all the week before. In the week without spending you might determine not to drive anywhere deemed not necessary, to eat inexpensive meals prepared at home (obviously), etc.