I generally have at least one 2.5 gal jug in the back of my Cherokee that gets rotated out apx weekly. I have the basics for emergencies in the vehicle (at least I do when they're not stolen and I've restocked from when they are...). The basics for me in the vehicle are:
Fleece blanket
space blanket
4 powerbars (or other brand)
firestarting supplies (Flint, tinder, matches, lighter)
Knife and Multi-tool
1st aid stuff
compass
Maps
sunblock
Hat, gloves
550 cord
Car stuff (tools etc)