[b3lha]

I am not a chemist, but I don't see any advantage in using heavy water for this application. A given mass of heavy water would contain less hydrogen atoms than the same mass of ordinary water.

Regarding water as a fuel, it is undoubtedly the best way to go, but there are two problems:

Firstly, separating the hydrogen and oxygen requires a lot of energy - more energy than you will get back from burning it (due to less than 100% efficiency). This is why car manufacturers are researching fuel cells which use the water more efficiently than combustion engines.

Secondly, the economic reason. Any companies that develop this technology will ensure that you will never be able to fill up your car up from your garden hose. If and when water powered cars are available, they will make sure that you have to buy special water at premium prices.

100 miles in 4 ounces doesn't sound right. According to a search, Hydrogen contains 2.8 times as much energy as gasoline. But water is only 11.2% hydrogen by mass. So 4 ounces of water is at best equivalent to 1.25 ounces of gasoline.