For a car kit, consider your realistic options in an emergency scenario.
Do you have provisions to stay warm and out of the elements? Stay hydrated and hunger satiated? Signal you are in distress or help identify your location?
I keep three rubbermaid tough boxes in my SUV. One is for on road emergencies. The other is for off road assistance and the last is my dookie hits the fan and I need to hunker down for a while box. Some things you want to include are a blanket or sleeping bag, tarp, candles (keep windows cracked), flashlight, paracord and zip ties, water container and pot, whistle, flares, fix-a-flat, tire repair kit, jumper cables (positive of live to positive of dead to negative of dead to ground. Reverse order to remove), kitty litter for use in winter, multi-tool, hatchet, crowbar, etc. etc. This is what I have in my car off the top of my head, I know there is more but I can't remember it right now.
An excellent source is Doug Ritter's site. He has a vehicle emergency kit list. Build yours around your realistic needs. Keep in mind that the 72 hour scenario of how long an emergency lasts is an average. Prepare to be out longer in case you don't fall within that average.
Good luck and keep us posted with your posts.