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In a diesel engine a constant throttle position means a constant volume of fuel is consumed per revolution. Only an engine RPM change will change your fuel used in GPH.
In a gas engine, there is almost no discernible correlation between throttle position and gallons of fuel consumed per hour. There are far too many variables to give a consistent answer, and most of the folks have already pointed them out.
I don't know how good Andyman's formula is, but his variables are inconsistent. The PA cannot both be manifold pressure and atmospheric pressure. The VAC is actually your manifold gage pressure. Volumetric efficiency varies with engine speed and throttle position. Even if the formula does work over a certain range of variables, it would only be meaningful at steady-state vehicle operation, and would have to be recalculated for a different set of parameters and would most assuredly yield a different answer.
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