If you haven't seen this before, you might want to check out wKiosk as a web browser option for your Mac based carputer. It is based on the Safari HTML engine and works as a full screen web browser. There is a free download to try it out, but if you don't pay for the full version, you just lose out on some of the advance settings which you probably wouldn't use, otherwise it seems to work fine.
Check it out at
www.app4mac.com. It looks pretty decent at 800x600, I have not personally seen it on a 7" screen yet. It has tabbed browsing as well. Not sure how many are planning on using their computer for web access. This is just another alternative that is designed to work full screen.