Any time you leave civilization you should have an emergency kit in your vehicle. At the very least you should always have a basic first aid kit, water, multi-tool, jack, tire iron, and spare tire. Toilet paper is also a darn good idea. In cold weather or high altitudes the minimum needs to include a survival blanket and candle. In hot weather include extra water and a hat.
In addition to these I have three kits that build on each other. The first is a basic kit for leaving pavement, winter driving, or even stormy weather. It contains bear spray, a fire extinguisher, basic tools (I usually also carry a full tool kit), a tow strap and shackle, gloves, whistle, toilet paper, emergency blankets, Rain Ponchos, a hood, a battery booster/power pack, jumper cables, emergency water, a light with extra batteries, an emergency reflector, and a larger first aid kit.
My second level kit is more survival based. It includes a tent, fishing equipment (lots of lakes and streams where I live), Survival food and water for two people for three days, good quality rain ponchos, various fire starting methods, tea lights, duct tape, sunscreen and after-bite, a bee-sting/snake-bite kit, a camp stove with fuel and pans, a pair of sporks, a flashlight with extra batteries, a crank light (no batteries needed), a compass, survival blankets, water purification tabs, a "life straw" water purifier, a folding shovel, paracord, a selection of knives and saws, toilet paper, a water jug, a tarp, a second first aid kit geared toward more serious injuries, and a splint kit.
My third kit, which I won't bother to show, is simply a big bag of survival food and water, toilet paper, toiletries, more survival blankets, more ponchos, and such. It has enough supplies to keep four people alive for two weeks.
Note that this is by no means everything I carry. I also have two levels of recovery kits depending on where I am headed, an extensive first aid kit I carry if I am alone or more than a couple hours away from medical aid, tire repair tools, a full tool kit, and so on.