If you are looking for a better fuel average sell the car , it wont give you more than 12-13 at best.
Do not use 10w40 , most mechanics try to put 10w40 in the car. Use 5w30.
Check if oil filter and air filter are of good quality.
Check the tyre pressure.
Check spark plugs.
Engine coolant level.
Check if the Catalytic convertor is clogged or not , run the emission test using OBD2 scanner. See the 02 sensors readings.
Check MAF sensor.
EGR valve and passage.
There are so many things , get them all checked.