Now matter how you look at it, it's going to be a tough sale without the support of upper management.
One successful combo I've found in building dealership web sites is to involve either the dealer principal or the GM (or both!) in the process from start to finish. Bottom line is web site designers know a lot about how to build web sites, and not much about how to sell cars. What better way to make a productive site right out of the gate than to ask the automotive professionals for input? Who is the target market? What appeals to them? What works in other media for you?
Once upper management becomes involved with the site, they tend to see to it that it's taken care of. They will allocate the budget and man hours needed to run it efficiently. Without fail, those sites we've built with this level of involvement are hugely successful.
The other successful combo of note is when upper management signs off on a site and allows one highly motivated individual at the dealership to run it and work the leads. In those cases, that individual gets ALL the leads that come off the net (and all the commissions
The down side is that once that person leaves the dealership, the closing ratios all but dissappear. All the internet closing knowledge walks out the door with that one person. That's great if you're that person! But bad from a dealership perspective :-/
The 30 day tracking system is a good idea. But do remember that not everyone who visits your site will contact you via email. Some dealerships track phone ups by placing special numbers on the site or utilizing an 800 number tracking system. A lot of people will STILL pick up the phone and call.. or even walk in, after visiting your site. Be sure the managers understand this, that the web site is generating more than just email leads.
If you're just one lone salesperson out there trying to convince a reluctant manager to buy in, it's really difficult to be successful online.
In fact, as a web design company, we don't take on any new accounts unless we have the full involvement/support of the Sales Manager or higher. We want to see our sites succeed, and if the dealership is being dragged, kicking and screaming, onto the internet, we pass.