Producing Hydrogen gas is not likely to increase fuel mileage in a vehicle,however it would have it's RE uses.
Of course, it can be used for cutting torches,replacing gas in heaters, stoves and water heaters..
It is also possible to use hydrogen production for energy storage. Generate gas w/excess electricity produced, store, then run an IC engine/genset when more power is needed.
My forays into hydrogen experiments aren't showing any advantages over other storage solutions though, the losses are too great at the DIY level.
It's still fun to experiment though.
