A Faraday cage controls the E-field, not so much the B-field. They help take the pulse to ground, but only if it is grounded. An old microwave oven with a very short wire to ground will work well.
I wouldn't worry about gasoline after an EMP attack. You'll be able to syphon it out of the hundreds of millions of dead cars littering the streets.
I wouldn't get anywhere near a city with that many cars in such a case.
Carry, excellent idea on the microwave oven substitute for a Faraday cage.