With dealer websites, for me the holy grail is a desktop site that can be sized down appropriately for tablets (iPads). Then a mobile layout that is done right. It's the hybrid approach like @craigh mentioned. So far I haven't seen a vendor hit a home run with the mobile site. I've seen one or two say they have, but they're probably one of the most un-usable mobile sites I've come across :/
Very interesting discussions going on here! Properly built responsive websites will work amazingly well on any device (including with touch interfaces). Someone mentioned that responsive sites need to download all the same CSS and image assets as on desktop.... this is not true if the site is built properly as each device can request it's own assets.
Anyway, check out a couple of these RWD dealer websites we've built. We've had nothing but amazing feedback from our customers and the results speak for themselves!
- http://www.sunautosales.ca/ (try this site on your desktop and then on your mobile device)
- http://www.cobourgkia.com/
- http://kingsautoltd.ca/