Sounds like futurama type of transportation but no I doubt it's energy efficient. If you try to get energy from gas, you'll always lose part of that. If you plan to keep all the gas from escaping, then the front end of the car will be hot (compressed gas) that could ignite stuff and the rear end cold (expanding gas). You may have to treat air so cold air won't make ice on the track. If one wants to save energy, get rid of personal vehicles and do more public transportation, especially in US. There's a light rail that's supposed to connect the major city to my place. The plan was decades back and money came in very slowly and now we have the light rail from the major city to half way point. I don't know when it will ever make to my place, which is supposed to be the terminal. On the other hand, light rail travels as slowly as cars, so what's the point? They travel fast in Europe, right? Here I've yet to see some US trains moving faster than cars.