accuracy of national new car inventory at cars.com and autotrader.com?

So I'm having a side discussion over MNL (My Nissan Leaf Forum • View topic - Elon Musk AND Bob Lutz on Charlie Rose 11/9/11) about the supposed supply constraints on Hondas and lack of inventories. A poster there pointed me to checking on cars.com and autotrader.com for new car inventory. Anybody know how accurate these are and how closely they represent vehicles actually in stock and not in transit, on order, etc.?

As an example, go to New Cars, Used Cars, Car Reviews, Car Finance Advice - Cars.com, select a make and model under New Cars and select All Miles under the dropdown and put in your zip. It claims there are 19,645 new Civics across inventory of all (US) dealers. Just for kicks, I tried the Chevy Volt, it claims there are 3277.

If accurate, the Civic # doesn't sound so bad since in November 2010 per http://www.hondanews.com/channels/c...ports-november-sales-increase-of-16-1-percent, they sold 16.5K Civics and 612 HCHs.

On New Cars, Used Cars - Find Cars at AutoTrader.com, click on Find Cars for sale, select Any Distance, enter your zip and select a make and model.

I am also more curious if there's a good place to look for metrics on # of days of supply for a given model. Some have said that's a better metric for sales vs. absolute sales numbers per month or year.