What is the cheapest way to travel in the U.S?

For public transportation, you described it perfectly: Cheap or fast, but you usually can't have it both ways. The exception is between large cities. For example, the bus between New York and Washington DC is usually very fast and goes direct. For travel in smaller towns, renting a car is usually the best way to go. Car rental is usually much cheaper than in Europe, and gas (petrol) is cheaper too. Often you can rent a car for around $30 a day.