fyi, this page says 1-2% of fuel is unburnt, consistently, before the converter. Take it for what it is worth. If you were able to reclaim that 1-2% you should see a 1-2% increase in efficiency at most (more likely %30 of %1-%2).
If you spend energy to make hydrogen inefficiently in the hopes of reclaiming that %1-%2, (by using %30 efficient combustion process and %60 efficient alternator and %50 efficient electrolosys) then you are bound to be worse off than where you started. Do you see a problem with this reasoning? The efficiency of creating the hydrogen is maybe %16 after all those losses, and you then pipe it into a %30 efficient engine, to help burn %1 of gasoline that didn't otherwise get burned.
What part of hho makes any sense? It simply doesn't add up. Calling me a naysayer is akin to calling someone who says 1+1 doesn't equal 3 a naysayer.
Fuel saving gadgets - a professional engineer's view