Whenever I fly the default 737, the learning center says that the max range SHOULD be 3000 NM, yet I can only get about 1500 NM out of the jet, is this a flaw in the default plane, or am I doing something wrong?

Yes,Change the Fuel Flow scalar in the aircraft.cfg to .75 or .55 (I can't remember which one). The effects should lead to a 6,000 FFPH. .