I listened to an interesting interview with one of the owners of a HHO conversion company.
He claims that the efficency increases are real and substancial. Reason is that when the extra hydrogen is introduced into the combustion chamber of a gas or diesel engine, the hydrogen molecules interfuse with the carbon fuel and the oxygen allows more complete combustion. The carbon fuels burn hotter and much more completely due to the synergestic effects.
He further explained that emmissions were radically reduced also due to much less unburned fuel out the exhaust. It seems that the hydrogen helps save fuel not so much by itself in the small amounts, but be enabling the gasoline to release much more useable heat to push the pistons and produce work. That is the trick, not replacing gas with hydrogen.
He also added that carbureator equipped cars, and fuel injection prior to 1998 produced the biggest gains, as the newer engines automatically retard the savings by forcing the engines to run on a very specific fuel to air mixture ratio. The older cars can run at a much lower fuel to air ratio due to significantly greater combustion and utilization of the fuel.
As for me, I hope it works. I think the guy was for real, and it makes sense. As for the big corporate "experts" saying impossible . . . of course they do. They might be right, but look at how many times we learn years or decades later they were wrong, especially when money is involved.