The other day a DUmmy, maybe this one, was all excited about development of a free, unlimited energy source: compressed air.
Actually, I can see some viability in compressed air, but it'll have to be coupled with an internal combustion engine. Most of your gas consumption is due to acceleration. If there was a way to compress air by normal driving, and, after stopping and then starting back, the saved compressed air kicks in and essentially launches you to the point where the gas engine kicks in, it would, IMO save a ton on gas.
One main drawback I could see is the maintenance. I had this conversation with one of my co-workers, a retired USN LtCDR Brownshoe who said it's very costly to maintain. Don't know all the details, was just thinking about it one day. Of course, I didn't think of this in some grand scheme to save a planet we're supposedly destroying
but out of selfish financial reasons.