as with any vehicle all the components have to work within spec and efficiently to achieve the best milage.
alot of variables can degrade economy:
blocked air filter, faulty MAF, stuck IAV, seized waxstat fast idle mechanism (throttle stuck open slightly), leaking injectors, plug gaps/fouled plug, faulty o2 feedback, major misfire, faulty coolant temp reading (stuck in cold start fast idle mode?), thermostat stuck open (wasting heat energy), incorrect ignition timing, collapsed cat, binding brakes, underinflated tyres, electrical drain (especially prolonged use of lights n heating during winter), driving style
check the easy stuff first like air filter, condition & gaps of the plugs (give hints of the general running mixture), clean the white oxide off the inside of the dizzy cap, check each wheel for binding brakes, clean the MAF & IAV with carb cleaner
next valuable tool is a usb consult data cable using nissan datascan software on a laptop to see all the sensor readings to the ecu. will be able to spot if the O2, MAF, IAV,TPS & temperature sensors within spec.
next is to synchronise the dizzy timing with the ecu using a scrobe preferably with datascan (software can force 15deg at idle and you simply match the crank pulley marking to 15deg with the strobe)
MAF, i think the facelifts use a bosch TB with replaceable MAF. there are other experts in here who can verify