Nobody has ever proven that HHO makes any difference to fuel consumption.
Many sites sell plans and salesmen make big commisions selling the idea.
The tiny jar of water and the small amount of bubbles are useless in an engine unless it is a ltttle lawnmower engine.
The amount of electrical power from the engine to drive the alternator to do the electrolysis to convert water into H2 and oxygen is higher than the power you get by burning it so the output power is reduced and the fuel consumption is increased.

 

Goes back to wanting numbers, we hear a lot of emperical evidence, but without data it's worthless. I don't totally dismiss it, but I still want to see some real data before I even start to buy in. Right now it's a cult following.