saw on news device that gets you 100 m.p.g. why are they not produced?
They are produced. You can find them all over the internet. These devices are heavily advertised, preferable using free news for credibility. Problem is, none of them offer much improvement, so while a few suckers will buy them, they can’t get mass sales. Were someone to make such a device, he could sell it to GM or Toyota for big bucks, so they could meet fleet mileage requirements.
When you study thermodynamics in college, you find that modern engines are very limited by the theoretical maximums. So, car makers improve aerodynamics, reduce car weight, use thinner oils, better combustion chambers to improve mixing, better ignition systems, more efficient transmissions and tires, etc, for tiny incremental improvements in mileage.
Everyone would like to believe that some "magic bullet" will let our cars burn water instead of gas, or get 100 mpg, but it just isn’t in the physics. No such thing as a free lunch …