A Curious Claim:

I received this in an email today from the campaign of Jerry Kilgore, who is running for VA Governor:

Senator George Allen routinely reminds us that a bumper sticker is worth $200 in free advertising or $300 if on a pick-up truck or S.U.V.

Why in the world would a bumper sticker on a "pick-up truck or S.U.V." be "worth" more in free advertising than say a mini-van or a sedan? Does it have something to do with attracting rural swing voters in Virginia to vote Republican? Or is it just some sort of joke that I don't get?