I claim the title -- ALL 48 contiguous states. My definition of "driving in" insists on a minimum of feet on the ground. I have filled up the gas tank, eaten a meal or stayed in nearly all of them. I've probably spent about 4 months on the road on perhaps a dozen different trips to travel in all of them.
I finished up last October with a trip to New England via South Dakota. I had intended to include SD the previous year, however the forest fires in 2009 in Yellowstone National Park closed the north and east entrances. By the time I got far enough east to hit SD I was many hundred miles north of it. This year it was just a slight detour (is any driving in the NSX a detour?) to head north instead of just going through Kansas.
At least 80% of these trips were on 2 lane roads -- why waste the tires on the interstates? It is much more fun -- and fewer cops -- on the "blue highways." I also get a much better feel for differences in different parts of the country. I also like to hit the mom & pop restaurants and off-brand motels, also. There are many more stories to tell of the different experiences this way, in my opinion. (Many overnights were with friends and family.)
I'm curious to see how many others have done this, or even come close. So far it is Greenberet that is the closest.