I don't think that it would break any laws of physics,
The first law of thermodynamics (compliments of wikipedia), " In any process, the total energy of the universe remains the same.
It can also be defined as:
for a thermodynamic cycle the sum of net heat supplied to the system and the net work done by the system is equal to zero.
More simply, the First Law states that energy cannot be created or destroyed; rather, the amount of energy lost in a steady state process cannot be greater than the amount of energy gained.
A pithy summation would be "You Can't Win".
This is the statement of conservation of energy for a thermodynamic system. It refers to the two ways that a closed system transfers energy to and from its surroundings - by the process of heating (or cooling) and the process of mechanical work. The rate of gain or loss in the stored energy of a system is determined by the rates of these two processes. In open systems, the flow of matter is another energy transfer mechanism, and extra terms must be included in the expression of the first law.
The First Law clarifies the nature of energy. It is a stored quantity which is independent of any particular process path, i.e., it is independent of the system history. If a system undergoes a thermodynamic cycle, whether it becomes warmer, cooler, larger, or smaller, then it will have the same amount of energy each time it returns to a particular state. Mathematically speaking, energy is a state function and infinitesimal changes in the energy are exact differentials.
All laws of thermodynamics but the First are statistical and simply describe the tendencies of macroscopic systems. For microscopic systems with few particles, the variations in the parameters become larger than the parameters themselves, and the assumptions of thermodynamics become meaningless. The First Law, i.e. the law of conservation, has become the most secure of all basic laws of science. At present, it is unquestioned."
If you want to improve economy in your truck, you have alot more potential for sucess trying to maximise atomation of the diesel and maximising the burn of your liquid fuel and save the HHO generators for the Saturday night infomercial.