Google recommends building your mobile site with a responsive design approach. This means that your mobile site uses the same URL as your main site, and serves the same HTML, but uses CSS to change how the page is shown on the device. Google says this is best since one URL makes it easier on users and Google doesn't have to crawl multiple pages on different URLs.
Their second-best option? Device-specific HTML. This means that the same URL is used, but the mobile version serves different HTML and CSS depending on the device that is detected.
All of this is a little technical, so make sure you talk to your car dealership mobile website provider to see which approach they are taking. You can read the full release from Google here: Recommendations for building smartphone-optimized websites.