I have a large plastic bin bag in my boot, aka trash bag in the trunk :-) which is ram-jam full of old cast-off fleeces, enough for several ill-fitting layers for two people, along with insulated hats and gloves. I never let my fuel go much below half. I've done this all my adult life and Mrs Steve thinks I'm mad. I've never had to resort to that gear but the whole lot of it is very light and I can squidge it down if I need space.