Hydrogen Generators are a excellent way to get crazy amounts of mpg out of your car. I've been following a guy that gets 30mpg in his old bronco.
It's a real science and a valid question. As this progresses I'd like to chime in more.
I cant speak on if hydrogen burns cooler than 93 octane however I know it burns more efficiently than 93. I started to build one for my VW jetta. (as if 35mpg isn't enough?!) however it got sent to the wayside after I got married.