Webwork - 12:06 pm on Dec 30, 2012 (gmt 0)
Not intending to be either flippant or rude I've got to suggest "don't go there" because your post of this issue/concern in the domain name forum (it's really an SEO issue, not a "domain name issue" and belongs in the Google forums) AND the sweeping nature of the question itself and how your frame it ("how will google handle a switch from real estate to autos") - suggests you will have a steep learning curve and serious challenge in making this idea/transition work.
IF Google's automated query relevance and ranking system has any functionality your proposed transition will be an uphill battle, worth neither the investment of money (in the domain) nor time/effort (in re-establishing the domain's/site's relevance and ranking for the new term).
IF the domain, in fact, has great inherent value and is available at an absurdly low price, then I'd acquire the domain for the sake of the domain's value itself . . undertandling that I'd have a lot of work ahead of me.