The confusion was introduced when Tim supposed that the very real “conservation of energy” law of physics could be reformulated as a sort-of “conservation of road vehicle power” law. Trouble is: there is no such law. The confusion was exacerbated by thinking of the truck “pulling” the car behind. It is doing no such thing.
Vehicles require power to move down a flat road at high speed primarily to push the air out of their way (and some to overcome wheel rolling resistance and other mechanical friction). As a truck moves through the air, it builds up a positive (over atmospheric) pressure in front of it and a negative pressure behind it because the air tries move out of the way from the front and tries to fill-in where the truck just moved out from in the back.
For the sake of argument, let?s suppose the pressure in the front is 20 psi and the pressure in the back is 10 psi when no car is present. The difference (20 ? 10 = 10 psi) produces a force that the truck?s engine must provide a counter force against to keep moving at a constant speed.
The car has the same basic problem as well with its own pressure difference, 20 psi in front and 10 psi in back, so its engine must also counter this force. But what if it comes up close behind a truck? The 20 psi high pressure zone in front of the car will now get pushed by the car into the truck?s 10 psi low pressure zone, resulting in a hybrid zone of pressure that is now different from the 10 psi that the truck would otherwise feel. Now it may be, say, 12 psi between the truck and car, while it?s still 20 psi in front of the truck, and 10 psi behind the car.
The result? The truck now feels only 20 - 12 = 8 psi of pressure difference, and the car feels only 12 ? 10 = 2 psi, whereas they both felt 10 psi of pressure when by themselves. Thus, the truck saves a little and the car saves a lot. BUT THEY BOTH SAVE!
?You can?t create energy out of nothing?, Tim said. True, you can?t. But that?s not what?s happening. What is happening is that the car by drafting the truck is using less energy in the first place by staying out of the wind and this is not the same thing as creating energy out of nothing. The energy savings represented by doing something more efficiently does not violate the laws of physics.