I've been having a think about your packing challenge, Cherri, and here are my thoughts, for what they're worth:
1. Use vacuum sacks to compress your rolled-up clothes - they considerably reduce the volume that clothes take up. (Our daughter Julia found these very useful when she took her gap-year in Oz: fitted her entire wardrobe and accessories for a whole year into a single 70 litre ruck sack!) Here's the sort of thing I'm talking about:
http://girlstravelclub.co.uk/catalog/product_info.php?products_id=189&osCsid=0 2. Miniaturize where possible: take a small sample-size tube of toothpaste, for example, instead of a standard-size one; shampoo sachets instead of bottles; cologne/perfume samplers instead of ordinary bottles, etc.
3. Pack lots of small bags, rather than a smaller number of large ones: it's easier to fit luggage into a car by doing so - you can more easily fit small items into awkward spaces than large ones! (Important: make a count of all your luggage items before you set out, and wherever you stay, count them all out and count them all in - that way you can be sure not to leave anything behind!)
I can't remember if your car has a hard top? But even if it has, I don't suppose they make roof boxes for sports cars, do they?
Also, don't forget that Rupert (Cheap Charlie) and Annette (Snork Maiden) make long trips in their Smart (see
http://www.ukusforum.com/index.php/topic,210.0.html for example), so they may well have a few tips as to how to fit things in.