For me, my longest nude drive would be the one I made with a friend of mine when he drove his jeep from Phoenix to Tucson via the back roads of Arizona. While he kept his shorts on during that trip since he was driving, I, on the other hand, got totally naked as soon as we left the city limits. Glad to say that I was able to remain buck naked for the entire trip. I must say that ever since that great experience, I enjoy driving naked whenever it is possible while keeping a pair of shorts close by!!!!
Longest drive for me was about 600 miles, 10+ hours, 8 hours daytime. 2 fuel stops were made with only a t-shirt long enough to cover the stuff. I wanted to be dressed that way when checking in to the hotel, but I was a little scared.
Second longest was 500 miles, 8 hours. This one was done going and coming. The outbound trip was all during the day. The return trip actually took about 9 hours with the first 3.5 during the day. All gas stops, as well as rest area stops,were made while only wearing the same t-shirt. The last fuel stop was late at night and included a trip into the store for restroom break and water/snack purchase. Incidentally, because of the late hour I arrived home, I was able to get out, unload and clean up the vehicle, all while remaining nude, and am still nude now, bringing my total time nude, with the exception of the short durations of required cover-up, to 16 hours.
The way I see it, most laws require that you not expose your "stuff", so a t-shirt that keeps your "stuff" from being seen should be sufficient, and not need shorts or anything.