Not an expert but can suggest few steps.
- Remove all unnecessary items from trunk.
- Ensure proper Tyre inflation and wheels aligned.
- Replace gear oil with ZIC fully synthetic and Use Liqui Moly gear oil additive.
- Do not use heavier than 10W-30 oil and use Liqui Moly Cera Tec oil additive.
- Clean O2 , MAP, IAC and EGR valve.
- Ensure fan auto with good coolant.
- Ensure correct heat range spark plug use (BKR6E11) with 1.1mm gap in each.
- Refer to service manual and note intake and exhaust valve clearance. Stand on the head of mechanic and ensure he adjusts accordingly.
- if equipped, have the catalytic converter cleaned by Sulphuric acid and nothing else.
- Ensure AC gas is not overcharged.
Hope it helps. There was great improvement in mileage of my cars after these steps.