This is a good comprehansive list that will serve you in many different situations, but isn't so large as to be unrealistic. Yes, it will fill most of your trunk, but you will still have room for other things. I'm only missing a few small things on the list that I'm working on getting, plus I have other items that are specific to my situation. This will give you a great start in being prepared while out in your car. You can add or take away anything from this list, but make sure to think through possible senarios before making that decision.

Items to Always Keep in Your Vehicle
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Items in passenger area

1. Cell phone battery charger
2. Flashlight/headlamp
3. Fresh batteries
4. Paper and pencil/pen/marker
5. Hand sanitizer
6. Road Atlas/maps of area
7. Life hammer

Items in trunk area

1. Air Compressor/portable jump starter
2. Shovel
3. Blanket
4. Brimmed hat*
5. Duct Tape, bailing wire
6. Water, rotate every 6-12 months
7. Food, snacks
8. Fire Extinguisher
9. Towels
10. Baby wipes
11. Toilet paper
12. Full size spare tire
13. Change of comfortable clothes, shoes, waterproof jacket& pants/poncho
14. Trash bags
15. Work gloves
16. Basic first aid kit
17. Fix a flat/slime
18. Tool kit
19. Car fluids/Wiper fluid
20. Road flares/LED hazard lights
21. Tow strap

Winter Items

1. Tire Chains
2. Gloves, warm hat*
3. Ice scrapper
4. Sand/cat litter/carpet remnants for traction in snow
   
   ​*Per person/ family member