Just a thought... Do you remove your front wheel to put the bike on your car rack? Even if you don't have to, you might consider popping it off and putting it in the trunk or taking it with you. That way if a thief takes your bike, he can't just ride away.

I have a friend who had bikes stolen out of his garage. He had about 6 or 8 bikes in the garage, and they stole 2. I asked him how they decided which bikes to take and which bikes to leave. He said they took the only bikes that had both wheels on them.

Like I said before, at least you have to make it tough for the thief!