Why you should have a Vehicle Emergency Kit, and what you should have in it.
Now lets get into what we suggest and why:
Water: Water is vital for life. Having water can also be handy in case you just need to wash your hands or if you have pets in a non-emergency situation. Always replace after use. Also you may want to carry a means to purify water, especially if your traveling far from civilization. Carry a pot to boil water/or a stainless steel bottle. You can boil water in the bottle to purify.
Shelter: for most regions your vehicle will be your shelter. What you carry will be different for the location your traveling in.
Desert regions-tarp and rigging lines.(you don't want to sit in your vehicle in extreme heat)
Arctic/Cold regions-Sleeping bags/extra jackets/blankets
Temperate Weather-Space blankets
Rain jackets or poncho
Food: Carrying non perishable food such as peanut butter, protein bars, and trail mix are excellent source of energy. They are also convenient. You don't want to take foods you would have to prepare. These like water are also good for non-emergency day to day situations where you may just want a snack.
1st Aid Kit: Carry the basic first aid stuff such as bandages and band-aids. Include toilet paper, soap, and hand sanitizer. Place any special medications that you may need in the first aid kit.
Tools: This section will be very location dependent. A small tool set, duct tape, multi-tool or knife, small folding shovel, jumper cables, fix a flat, jack/tire iron, flashlight, extra batteries, chem-lights, and flares
Optional tools: tow strap, ice scraper, and snow chains.
Navigation: Always have a Map/GPS is optional.
3 methods to start fire: Bic lighters & Black Scouts fire steel. Flares can also be used to start fire
Weather can be unpredictable. A quick trip could turn into a life or death ordeal. Having this gear may also give you opportunity to help someone else in need. This is a basic list and should get you through most situations. If you plan on going off grid I would suggest also adding a Personal Survival kit and a Pack to place all the gear you need if you have to self-rescue.