Here is my update if someone is interested.
Last year average is now 5.8 l / 100km. (slowly going down) (40.5 US MPG)
last mods done:
- ScanGauge installed
- Mobil ATF SHC transmission fluid to manual gearbox works fine in astra G manual gearbox (not yet problems)
- Roofrack removal done (OEM caravan racks)
- Mobil 0W-30 fueleconomy oil in motor (not yet problems)
- Higher frontgrill blocked and motor air intake under hood. (intake air 8 Kelvin higher helps in cold conditions)
- Cleaning at least 20kg of extra stuff.
Next mods ??
- something with alternator (double size pulley ? )
- aerodynamical mods mostly under the car
- led bulbs to registerplate
- finishing the roofrack removal (covers)
- finishing the higher frontgrill (something else than PVC tape
)
- lower suspension ?
- no need for new tyres. Economic driving makes tyres last almost forever.