I suspect the purported safety of a car in a storm is that the body of the car offers a more direct ground path and may even act as a Faraday cage.
The Faraday cage effect is more significant than the path-to-ground effect. Lightning has a tendancy to take all easy paths, and even if you're next to a metal car frame, you're still a pretty good path, by lightning's standards. Of course, it does help, so being in a convertable would still be better than just being out in the open, assuming that the windshield frame is metal (of course, you'd ruin the upholstery, but that's another matter). The effect from the tires is completely, utterly, totally negligable. Air is a much better insulator than rubber, and lightning laughs at air.
Being on a bike would probably be worse than just standing: The lighting only has to go through about a meter of flesh, and then a meter of metal, instead of about two meters of flesh. Unfortunately, the flesh that it does go through includes the head and the heart, so you'll still fry from it.