Run out of petrol? These days I make a point of carrying a spare can.

"I always fill the tank - I've never understood the logic of not doing so..." The logic of not filling the tank for me is that it saves me from getting upset about it costing more than it did last time. If doing it that way worked out marginally more expensive I think it'd be worth the difference. But since I generally buy petrol at the same time as doing the shopping it doesn't in fact involve any extra mileage.