There are many ways to mod the ECU. But if you cant, you can install a AFPR(adjustable Fuel pressure Regulator) Stock setting is usually 43psi. If you cant get the ECU to reduce fuel input. Cut the fuel supply manually by reducing the incoming fuel pressure. IE lower to 35psi. YOu can get 1 on EBay for $20.
Hows it work? An FPR acts as a constrictor on the return line to your fuel tank. aka like a thumb on the end of a garden hose. higher FP= more fuel better performance, (12.5 AFR) is peak performance. Lower FP = better mpg. DONT go 2 lean though.
This should only be done w/ an AFR(air to fuel ration gauge) and a EGT(Exhaust gas temp) gauge is also a good idea Anything above 1600F and you will be in trouble... eventually sooner or later. 15-15.5 AFR reading is slightly lean but ok anything higher and youll risk damage like melting your cat if you have 1 or warping valves ect.
Now if you wanted to just use an AFPR to enhance your mpg you can. But just remember since the HHO gives more O reading to the O2 sensor I would add 0.4-0.6 to AFR gauge to get an accurate reading.( If you have a higher LPM based on your engine size you could add more, but the rule of thumb is .5L HHO/ 1Lengine) The ECU shoots for 14.7 AFR (stoich) but really this same gas ratio should be about 15.2 w/ HHO. So simply reduce your Fuel pressure until you reach this reading. This setup will cause a check engine light(CEL), but the good thing about it is that the ECU wont be able to add anymore fuel once put in closed loop, just lower the FP till the readings are as state above, about 15.2 and you should be fine.
Check the spark plugs after a week or 2 and make sure they are not white. White indicates a lean cond. Tan is the proper color.
NOTE: only adjust the FP when the car has fully warmed up. It will run richer when the car is warming up. This is not a bad thing.
If youre worried about passing an inspection. Just raise FP back up erase CEL, pass, then turn back down.