Question: reasons why mpg is low?
Answer: Driving Style and Circumstances. One of the most common causes of poor fuel economy is the way you drive and the trips that you make. ... 
Bad Oxygen Sensors. ... 
Faulty Thermostat. ... 
Faulty Fuel Injectors. ... 
Engine Misfire. ... 
Faulty Spark Plugs. ... 
Clogged Air Filter. ... 
Wrong Oil Viscosity.