A few years ago there was a "race around America" race. The idea was that you started from say NYC. You headed south to Florida, west to LA, north to Seattle and then back to NYC. You drive 24 hours per day and only stop for gas. The total distance is about 7400 miles and you can do it in 107 hours driving the speed limit or 98 hours (4 days +) if you speed just a bit. Can you imagine renting a car and returning it 4 days later with 7400 miles on it.

One team did just that which might have been a reason that the rental companies put some sort of limits on total mileage.