text: see the following question related to physics and energy-? i am not a physicist, so please blow me out of the water with an answer, but... if i wanted to design and build a car that would conserve energy, no matter what the primary fuel was, couldn't i add some things to the design of the car to create secondary energy that could then be used as a power source to use to extend the mileage?  for instance, why couldn't i add "aluminum pinwhhels" all over the leading edges of the car that would generate a whole bunch of power during roadtrips, and this energy could be stored in some type of battery.  or if the primary energy source was battery-electric power, couldn't a combination of road friction, wind power and solar power from panels on the hood, roof and trunk areas generate more than enough power to keep the main battery always at full charge?  i know this vehicle wouldn't be sexy, but couldn't it work and be a very energy-efficient vehicle, costing zero to operate?  where am i off base?  let me know because i want to start production. there are two reason's you are off base.  the really physicsy reason is that the second law of thermodynamics forbids any 100% efficient engine (in your case, the whole car is the engine).  without going into too much detail, this is because energy is always conserved in the universe, and any engine loses energy to heat, at least by friction.  the more understandable reason is this: taking your pinwheel windmill generators as an example, the work that they provide (to charge up the battery) does not come from wind, it comes from the car moving, which comes from the car's engine.  therefore, all the energy going into the battery is extra work the car has to do.    you can, however, save some energy when you slow down the car, you can return some of its moving energy (kinetic) into a battery.