Here is a list if things to check that can cause bad gas mileage
Malfunctioning Fuel system. Fuel pressure too high or too low.
Leaky injector or fuel pressure regulator.
Bad or lazy Oxygen Sensor
Sensor Inputs.. Bad or Skewed Coolant
temp sensor, mass air flow sensor, Map sensor, Throttle position sensor
— Transmission shifting
Anything in the drive train. Bad calipers (hanging up), binding drive shaft or bad wheel bearings or even low tire pressure
Engine mechanical.. Valve timing off by timing chain or belt, restricted exhaust
, low compression and or valve train problems... valve mot sealing properly, bent valves carboned up valves or motor
Engine Maintenance - Change your oil, spark plugs
, and air filter regularly. And go to a repair shop if your "Check Engine" light comes on-a faulty oxygen sensor could be the cause, lowering your mileage significantly.
Engine Oil - Use the grade of motor oil recommended by your vehicle's manufacturer. Motor oil that bears the American Petroleum Institutes "Energy Conserving" performance label contains friction-reducing additives that save gas by letting your engine work more easily.
Fuel/Intake System Restrictions - Dirty air filters, fuel filters, and fuel injectors
make your engine work harder to generate the same amount of power at the wheels, thus wasting more of its own energy and reducing mileage. This is probably the most common cause of gradual fuel economy loss because, unless the problem is severe, computer controls can usually compensate such that the average driver doesn't notice any issues. This is why it's important to stick to maintenance schedules, even if it seems unnecessary at the time.
Low tire pressure allows a greater portion of the tire tread to contact the road, increasing friction and sapping power (it also causes accelerated and abnormal tire wear). Wheel alignment problems cause similar issues by forcing tires to "drag" across the pavement to some degree, rather than roll smoothly. Less commonly, malfunctioning braking or all-wheel drive systems can cause additional drag at one or more wheels, wasting power.