Basic kit
This kit is intended to aid you in getting help, signaling your car's presence to other motorists, and tackling simple challenges.
Cellular phone - We don't recommend that you talk on a cell phone while driving, but in an emergency, this can be the single most valuable component of your kit. Keep a car charger handy. This device plugs into the cigarette lighter or other power point in the car and charges the battery of your cell phone. When traveling, it's best to leave your cell phone on. Emergency tip: If you have to dial 911, remember that your location and phone number aren't always available to an emergency operator when calling from a cell phone. So give the operator your number and any information you have about your location. Ignore any "no service" messages on the phone and try the call anyway. If you have trouble connecting to 911 from inside a car, get out if possible and call from the side of the road. That may help you get a better signal.
First-aid kit - Choose one that allows you to treat a range of problems, from small cuts or burns to ones that require major bandaging. We also suggest you get familiar with how to use the kit before you need to.
Fire extinguisher - A car fire can start from something as simple as a wiring short circuit or leaking oil. You should get away from a vehicle that's on fire as quickly as possible. Still, for extra security it's good to keep a fire extinguisher in the car that can be used in any emergency or to quickly dose a small flame that's just begun. The quicker a fire can be put out, the less damage it will cause. Multipurpose dry-chemical fire extinguishers are available in a variety of sizes. We recommend carrying a compact unit that's labeled 1A10BC or 2A10BC.
Warning light, hazard triangle, or flares - If your vehicle is stuck on the side of the road, it's vital that you give other motorists as much warning of its presence as possible, especially at night. Look for a battery-powered warning light that can be placed far from the vehicle. Reflective hazard triangles and flares are also effective and don't need batteries.
Tire gauge - This should be used on a monthly basis to check the inflation pressure in all four tires and the spare tire. Because the ambient temperature affects tire pressure, it's also advisable to check the pressure after a significant change in temperature. See our latest Ratings and buying advice on tire pressure gauges.
Jack and lug wrench - Almost all vehicles come with these items for changing a tire. Refer to your owner's manual on where they're located in the vehicle and how to use them. Models that come with run-flat tires do not have a spare tire. Run-flat tires can be driven a limited number of miles with little or no air in them. They have very stiff sidewalls, which provide support when the tire is deflated. Learn more about the warning signs of imminent tire failure.
Foam tire sealant or a portable compressor and plug kit - For minor punctures, a foam tire sealant can get your vehicle back on the road quickly. Only use it in an emergency, however, as many tire shops will refuse to repair the tire because of the sticky residue these sealants leave inside it. Be sure to choose a sealant that's labeled as non-flammable, and don't consider this a permanent fix. A portable DC-powered air compressor can also be used to inflate a tire--and is especially handy for one that suffers from a slow leak. To fix a puncture, however, you need to have it professionally repaired.
Spare fuses - If you experience an electrical problem, your first check should be for a burned-out fuse. These are easy to check and replace by referring to your owner's manual. Keep an assortment on hand of the proper type for your vehicle.
Jumper cables or a portable battery booster - Jumper cables are easy to use as long as you have a second car available to provide a jump. Refer to your owner's manual for instructions. A portable battery booster eliminates the need for a second car.
Flashlight - This can be critical at night. Choose one that is bright and weatherproof. In addition, a flashlight with a magnet, flexible mounting system, or a stand will free up your hands for other tasks. Also, have extra batteries and a bulb available.
Gloves, hand cleaner, and clean rags - Even the simplest jobs can get your hands dirty. Having these on hand will help keep that dirt from getting on your clothes or your vehicle's interior.
Auto-club card or roadside-assistance number - If you belong to an auto club or roadside-assistance program, be sure you have the necessary information in your vehicle.
Camera - Following an accident, this lets you record the condition of your vehicle and other vehicles for insurance purposes. A cell phone camera can also work.
$20 in small bills and change - Keep this available for miscellaneous use. And resist dipping into it for a spontaneous ice cream cone on a hot day.
Pen and pad of paper - This can come in handy for a range of uses, from leaving a note on the windshield should you have to leave your car to jotting down information after an accident.
Additional items for winter driving
For the cold, wet conditions of winter, you may need additional items in your emergency kit, especially if you travel in remote areas or in severe conditions.
Windshield scraper - Good visibility is your most important safety item, but persistent snow and ice can build up quickly and make it hard to see. A long-handled, soft-bristled brush can also come in handy.
Tire chains and tow strap - Familiarize yourself with how to put the chains on your vehicle's tires or attach a tow strap before you need to do it in cold and possibly dark conditions.
Blanket and winter hat - If you run out of fuel or if your battery dies, the vehicle won't be able to provide heat. A blanket and hat can help keep you warm if you have to wait for a long time in cold conditions.
Chemical hand warmers - These small, inexpensive packets are available at ski shops and sporting-goods stores.
Small folding shovel - If you get stuck in snow, this can be a vital tool. A folding camping-style shovel will require more digging effort than a longer-handled shovel, but is more convenient to store in the vehicle.
Bag of cat litter - This can help provide some traction on an especially slick road surface.
Source: http://www.consumerreports.org/cro/2012/03/roadside-emergency-kit-what-to-carry-with-you/index.htm