Perhaps you meant Oxyhydrogen, but that is a mix of O2 and H2. Again, there is no such thing as HHO. Santilli proposed magnecules to exist, HHO being one of them, but no one has ever seen an HHO molecule, and Santilli is regarded as a proponent of false theories. Let's call it Hydrogen and Oxygen, so at least we are in the realm of reality.
Here is a link to proper scientific studies of adding Hydrogen to different engines. Granted, in Diesel engines it does help completely burn the mixture, but as stated in the article, it is highly unlikely that it is possible to increase fuel efficiency if you are generating the hydrogen from water. It has never been demonstrated in a lab. I am sure that the folks at the Jet Propulsion Laboratory had much more money and expertise and did not even bother trying this for obvious reasons, it won't work.
Adding hydrogen is very effective in reducing emissions and is the most useful application for this technology. There is a government evaluation program for any product that can aid fuel economy for cars or trucks that will give you millions of dollars if you can prove it works, however there is no product using "HHO" gas that has even applied for this. Why would they turn down money, unless it's a scam.
Wow, driving slower gives you better mpg?!?! Who would have thought.
All the major car companies invest billions of dollars into improving fuel efficiency, which helps them sell their cars over their competitors. Don't you think they would add cheap products like these if they worked.