I do remember reading somewhere that one of these silly devices is actually real, I think it only worked for diesel, and it provided a very small boost in fuel economy by somehow causing the engine to combust the fuel more fully, emitting less unburnt fuel in the exhaust, thereby getting more "bang for the buck". But it was nowhere near a 33% fuel-mileage gain.
I highly highly doubt it. Maybe something really negligible such that the device would never pay itself off.
If that were true the thing would be mass produced in all diesel trucks and whatever. It'd be standard.
It's not like they're hard devices to design, or a new idea, and if it's not cost effective to produce shitloads it's not cost effective to produce hundreds (at a higher R&D and unit cost).