No, aluminum is a better conductor.
Comparison of various metals on WikipediaTires are not made out of just rubber. They can have steel belts, and many other materials. I'm having trouble finding a good reference explaining the physics of it - but in a strong eletrical field, they become (relatively poor) conductors... Found this though, which mentions it in passing:
How Stuff Works entry on lightningAnd yes, you're safe inside your aluminum trailer or steel car, that is, assuming they don't catch fire from the lightening strike, but then it's the fire that gets you.
Faraday Cage on Wikipedia
