Drew is right. The only true measurement has to come from the providers analytics directly. The advantage of sites like Alexa are they're typically uniformly inaccurate-- so are best used to measure the difference between website's influence than actual numbers.
Cars.com, AutoTrader.com, CarSoup.com etc., all can provide site or market wide analytics if you ask them. That's the best place to start. It never hurts to ask what they're doing for you-- for example how they're improving their sites to ensure that the number of leads per traffic they're generating goes to you and increases over time.
A couple notes on that...
Cars.com has extensive consulting and experts that guide their analysis and improvements of the site. They're a great dealer resource and very, very digitally savvy (my personal favorite). They're very consumer focused with dealers in mind.
AutoTrader.com has an automated tool that allows their site to "learn" from user behavior to serve the best performing content on their site which in theory increases the amounts of leads "passively" for dealers. However, they're very business focused, with consumers in mind.
CarSoup.com is dealer and market focused, with business in mind.
