Type that into google, and you will actually get a lot of adds for sites that are offering "conversion kits". Since google doesn't give ads for free, that much mean that some folks are actually buying these products. What's even more concerning is that some of these products have apparently been featured on actual TV news shows (not just infomercials made to look like TV news, there are multiple news videos yotube). I'm not journalist, but shouldn't folks doing news do some filtering for things that at least make some sense?
If we were actually able to suspend the laws of thermodynamics, here's a question to ponder: If you could buy a car like would actually run on just water, and it was just as reliable as a real car, and it was just a powerful as a real car, how much extra would you be willing to pay for it? From a purely economical standpoint, probably not much more than $20,000. A typical car will work for about 120,000 miles and will burn about 4000-6000 gallons of gas. At today's rate of about $3.50/gallon, that's about $14,000-$21,000 over the life of the car. Back when gas was $1.00 a gallon, that was obviously much less. Of course, there's a niche market for people who would pay for the good karma of helping the environment, or wanting to buy less foreign oil, but that's a small niche.
Back in the real world, it's very hard to get a car to run on less fuel while maintaining the same power and reliability. You can see why most folks never paid much attention to the fuel efficiency of their car. That $14,000 number is an upper bound for what auto manufacturers could make by making their cars more efficient. Even that $14,000 number is in danger because the price of gas could fall back down by the time the technology was able for production.