As a general rule, TV reception in a moving vehicle is lousy.
I'm certain that HD signals would be worse, since the signals are digital.
i just found out about this cool lil product which is a mini digital hdtv usb receiver. basically, what it is, is that its a hdtv receiver in usb format, and you can hook up an antenna to get the free air hdtv signals. i was wondering if anyone here has heard of it, or have it in their carpcs. it seems pretty cool and it seems like it would work in the carpcs. its kind of like a tivo, so you can also record your shows. heres some information about it too, plextor.