Water4gas

An experiment: set your carb to run your engine at 2,000 r.p.m. Take a bottle of propane (soldering tank), put the nozzle over the carb intake, and turn on the gas. If your r.p.m. increases, then you are producing more power. This can also be translated to cruising at a specific speed on the road; you would require less throttle to maintain that specific speed.

Then, there is the question of how much energy Brown's gas provides as compared to the same volume of propane.

As far as the cost of the produced Brown's gas is concerned, you first need to determine how much improvement in mileage/power you are getting from it. Most of the electricity potential produced by your alternator is not used during cruising condition, anyway. A more powerful alternator will draw a little more engine power to turn, but this might be relatively tiny.