my old landlord used to make and experiment with his own systems. he was just starting and the gains were not huge but he did get an extra 20 mpg, on a Saturn and neon, from the system he had built. supposedly now he is getting more. one big part is not just the HHO generator but being able to control the fuel system in the vehicle, to lean out the fuel mixture. (the o2 sensor will read the extra oxygen fron the generator and actually add more gas to the mix if there is no way to control it). he also changed all the fluids to synthetics, ran the tires at just above the max pressure and made this thing that heated the fuel before it was injected. and of course driving easily. I am sure his other mod's could have had helped with the overall gain. his website is
Hydrogen-Boost I am not spamming, he has some information and videos on there that may be useful, there is some usual sales BS but you can make your own opinion.