Googling "Faraday cage" yields a lot of interesting results.

Not just on EMP but also lightning strikes. Previously, I'd assumed a car was a refuge in a lightning storm because of the rubber tires. Nope. And my Honda Element's plastic panels reduce the Faraday protection. Dang. Argues for stocking up on bike tires and tennis shoes.

Don't know anything about this person or their qualifications, but they are concise:

http://preparednesspro.wordpress.com/2009/06/18/emp-101-part-iv-faraday-cage/

a car is NOT a Faraday cage sufficient to withstand an EMP incident. It has some similar components, yes. Most cars made today consist of fiberglass and disjointed parts, not a continuous metal material. In addition to that, they are on tires. Tires on a car do NOT serve as grounding. Folks are simply getting an EMP strike confused with a lightening strike. Now, IF you had an old fashioned car that was made of metal, that had its tires removed, that was also attached to an Iron or copper pole and that was ALSO on dirt—not gravel—then yes, you may have a car that doubles as a Faraday cage.

You can have an instant Faraday cage with a galvanized trash can or a large stock pot like they use in restaurants. (Be sure to clamp the lid down. Remember—continuous connection is key. Since Faraday cages are not fool proof, depending on the strength of the pulse, I would recommend burying such containers 2 feet under the ground, storing survival electrical and battery items. (Including batteries).

Be selective in what you protect. It makes no sense to protect a cell phone, for example, as the cell towers will be useless. If it were me, I would protect radios, communication devices (such as a HAM radio), batteries and all of their respective tools, thumb drives loaded with all of my vital information, and a laptop.


National Lightning Safety Institute

http://www.lightningsafety.com/nlsi_pls/vehicle_strike.html

Electrically speaking, at lightning's higher frequencies, currents are carried mostly on the outside of conducting objects. A thick copper wire or a hollow-wall metal pipe will carry most of the lightning on outer surfaces. This phenomenon is called "skin effect." The same holds true for lightning when it strikes metal vehicles: the outer surface carries most of the electricity. The persons inside this steel box can be likened to protected by a partial Faraday cage.

If the car is made of fiberglass (a poor conductor) or a convertible, skin effect principles may not work. (Corvette and Saturn owners, please note this fact.)

School Buses. Metal buses are good Faraday cages.