got to say that Holden's posts have been absolutely spot on, theres 100s of years of experimentation and proof to back up what he's saying. might be a good time to summarise:
the booster will draw current, which increases load on the engine.
the hydrogen burnt in the engine provides less energy than the fuel used to produce it.
if the burning fuel is converted into kinetic energy at the same efficiency as pre-hydroxy, the engine will be less efficient than before and mpg will decrease.
so for this to work (ie increase the engines efficiency,) the fuel -> kinetic energy stage must be more efficient to overcome the losses from hydroxy production, and then some to provide a net increase in efficiency (more mpg)
so what we're looking for is the hydroxy improving the efficiency of the combustion stage, by quite a margin
modern engine designs use a range of techniques to achieve better efficiency in the combustion stage (direct injection, combustion chamber/port/manifold design, ECUs to control fuel/air mix, timing etc under specific loads)
older engines by comparison can be very inefficient, so there may be room for improvement. If by chance the engine would benefit from a faster burn, you'll see an improvement.
btw I've reduced my fuel costs by switching from 95RON standard to 97/98RON super unleaded, costs 5% more and mpg improves by 15-20%
ymmv