Recently we tested a rather new truck (build 2010)
We outfitted it with a HHO generator set of ours
and we let it run for two weeks just to check for the drivers normal behaviour pattern.
after these two weeks we switched on the generator
the next trip the driver didn’t know what had happened to the truck just after an hours drive he noticed that it appeared more responsive to the foot pedal and after, for the rest, a normal week while filling up the tank he noticed the ultimate result
the meter of the gasstation stopped and after some basic math it came out that he had used aprox. 25 % less fuel that week
We stil have to poor over tthe data from the accumulated trips to get the exact results but so far so good.