So I seems that high temperatures helps the mileage. Too bad the carr cools to fast once it stops working...
This time I drove with no grill block, only with 3,5 on the tires, and I was able to repeat the trick
I traveled mostly in highway, trips with more than 60km in distance (about 500km with +3 passengers, and about 250 with +1 passanger), and very few short trips (less than 20km).