EdwardS wrote on Feb 24^th, 2012 at 2:39am:

Does anyone know if FSX actually takes into account the planes weight when figuring fuel burn? For example if I burn off 2,000 pounds of fuel will my fuel burn over the next hour be less because the plane weighs less? I have never noticed FSX doing this.

The only thing that I have noticed with most (if not all) aircraft, is that as you burn fuel off, the aircraft gets lighter - so its speed increases.
So the fuel burn seems to remain the same, while the plane goes faster.

Mark