I will make the question clear, if a car is moving at a constant speed (constant RPM) no acceleration or decleration (no going up the hill or down the hill).

Through some research I found this equation: Fuel consumption per minute = (RPM/2)*(Engine size in Liters), however the number it gave was really out of scope.

The other equation which is the one I got from this thread which is

Fuel consumption (liters/sec) = RPM * injector open time * injector flow

For the this equation, I assumed that injector flow is the engine capacity and it gave me good results but I still need to know the source for the equaiton.