In December I started working on one for our family, I still do not have it to my liking but I did find out in a small way just how important one is to have.
A few weeks ago we left Florida to meet our daughter in Louisiana to pick up our 3 year old grandson. When we left Florida, the temps were in the 70's , we were dressed accordingly and I failed to check the weather before we left. I just assumed that since we would be traveling southwest that the weather would be much the same. Wrong..... The further west we drove, the colder it got and by the time we got to Baton Rouge it was bitter cold outside, pouring rain with a little ice mixed in. In our bag I had packed a set of sweat suits for the three of us and we were ever so grateful to slip the sweat shirts on over our clothing for warmth. Inside the bag were also snacks of Vienna Sausage, beef jerky and granola bars and small bottled water. We had snacks and drinks to keep our little grandson happy without having to stop and get in and out of the icy rain.
I would not suggest storing your supplies long term in a vehicle. If you have cans with pop lids they can explode in extreme heat, the hard candies and candles can also melt and you know in Florida what temperatures a vehicle can heat up to when the sun is out.
During a hurricane evacuation we have been fortunate not to have ever been caught sitting hours and hours in non moving traffic but there were thousands who were. If they had their 72 hour kits in their vehicle, it made life much easier.
The following is a handout by LDS I found on the web. It is a good guideline to go by and can be adjusted to fit you family needs.
72-hour kits - What is needed?
1 sturdy backpack per person (or rolling suitcase), or some rubber maid bins (preferably with wheels). You pretty much need something that you will be able to grab at a moment's notice and throw into the back of the car
1 change of clothes per person
1 set of scriptures per family
1 flashlight per person
small first aid kit
personal documents (copies of birth certs, ss cards, wills, patriarchal blessings, insurance card copies, etc). It's also good to have copies of these to send to a family member who doesn't live in your same area
2 gallons water per person (this is a MINIMUM. 3 gallons per person is ideal)
$100 cash per family (make sure you have some change and small bills)
ax & shovel (obviously, this will probably not be kept WITH your kit, but have them available)
battery powered radio and light
small sewing kit
here is an example of food to store in your kits. Remember to rotate every 6 months!
1 lb dried fruit or trail mix per person
1 pkg soda crackers AND graham crackers per person (there are 4 pkgs per box)
2 liter bottle of orange juice or similar per person
4 granola bars per person
1 can meat per person and 1 can beans per person chicken, tuna, etc and chili, etc.
2 sticks beef jerky per person
1 pkg chewing gum per person
2 packets hot chocolate mix per person
2 instant soup packets per person
1 roll of toilet paper per person
1/2 lb dried milk per person
hard candy / lollipops
2 light sticks
hand warmer / canned heat
prepare blankets or sleeping bags to be accessible at a moment's notice
1 small toothbrush per person
toothpaste, soap, and shampoo (travel sizes are great)
silver foil emergency blankets (one per person)
small entertainment (crayons, card games, etc) and paper/pencils
camp stove or portable BBQ & fuel
work gloves (2 pair per family)
mess kits or disposable plates/cups/bowls/utensils
candles, holders, matches