Just use your car computer that reports instantaneous fuel economy, or if your car doesn’t have one buy a ScanGauge and connect it into your OBDii port. RIGHT?
The problem with OBDii gauges is they rely on MAF voltage and are calibrated to provide fuel economy using a fixed air fuel ratio. The Volo decreases the air fuel ratio and since the MAF can't detect the change from standard the reported fuel economy remains the same.
It is a bit more complicated than that, and you need to be scientific about it.
The key to accurately comparing fuel economy is in keeping the conditions exactly the same with the only variable being HHO, and doing an actual measurement.
It is nearly impossible, but any changes in conditions can affect mileage, so do your best to control these:
We recommend limiting your fuel economy comparisons to only highway driving conditions because with city driving traffic patterns are much less controllable.
We also recommend identifying a round trip course and topping off your fuel tank at the same station each time you start the course. Try to drive exactly the same each time you run the course practicing the same acceleration, speed, deceleration, braking, idle habits, and use cruise control if you have it. It is amazing how much we can change our mileage by simply driving less aggressively.
A Note Concerning the Volo Chip & HHO
The Volo chip alone will increase mileage without HHO. The reason is the Volo is designed to reduce fuel consumption at the expense of power and almost starves the engine in certain situations, and HHO gas increases the energy content of combustion effectively restoring the power while preserving fuel economy.
When you first connect the Volo to power (or reset it) it takes a while for it to learn how your engine is reacting to the level of HHO it is being feed (as long as 120 miles for best results). Once it is trained it will remember until power is lost or until its reset and it will then have to calibrate again (note: re calibration is required when the engine is tuned or engine components are replaced).
When you want to measure a run without HHO you must disconnect the Volo from power in addition to turning off the HHO generator. And, when you reconnect the Volo you will need to allow time for calibration before you start a mileage run.
A Good Mileage Comparison Route
I make regular trips to Athens and from here it’s 250 km each way. Before HHO I could never get there and back with a full tank... I would always stop and put 20 euros in to make it back home. The route I take has only two highways. With HHO I get back on a single tank and usually another 40km of city driving locally. I have a Korean car with 4th gen engine and HHO works excellent with Volo chip.