It's called a run down test. You accelerate to a certain speed, and then coast and the way the car slows down is where you get the data from.<p>It's not a new idea. Teams including Audi's Le Mans squad use it. Teams have rented the Shuttle landing facility at Cape Canaveral to do the same thing. The advantage of the Tunnel is there are less variables to contend with such as things like wind.