If we have the cc of the engine, and its running speed, how can we calculate the mass of gasoline in milligrams sucked in each intake stroke and the mass of gas consumed in a given distance, based on its mass and volume?
Just a thought of a way to determine what you are looking for...
Your injectors are rated for a certain flow at a certain PSI. Most cars have a Fuel Pressure Regulator to keep this pressure constant relative to the pressure in the manifold. You should be able to find this flow rating for your vehicle's injectors.
Find a way to 'watch' how long the injector is open. This is going to happen is the millisecond range, I'm thinking an arduino using the injector power wire as an input, constantly polling and recording/outputting changes. Not sure if the arduino would be fast/accurate enough. The car is 12v and the arduino needs 5v, so you would need to drop the voltage somehow and use a diode so your reading doesn't effect injector performance.
Once you have a flow rate and how long it is open, you can calculate how much fuel is injected.
How about filling the tank, driving it however far and re-filling the tank. The greater the distance travelled, the more accurate the amount of consumption per unit of distance.