A family member tried something similar on a Jeep that I then took over. It used Potassium hydroxide to generate the hydrogen and fed it into the air intake. I did not make scientific calculations but in general I saw no change in MPG after I removed it. In addition the caustic quality of the solution left some damage to the paint in engine compartment and the throttle body. Luckily the Jeep was pretty impervious and didn't seem affected by this. Can't really recommend this unless someone shows me some real world mpg from before and after.
Plus the old analogy - you can't create more energy applies here. Some of these devices use energy from the alternator to generate the hydrogen. So you are using gasoline to increase the alternator output to generate hydrogen to use less gasoline.
Can't get it for free.
