Wow!!! Congrats it is awesome
I am a bit jealous as I cannot for the moment go much under 4l/100km and my instantenuous fuel consumption at 62mph (real speed, so about 105 at the gauge) is more about 4l/100km. But it is true I haven't used all your tricks now and I live in the mountain. Even with much care, it can't be helped that a lot of gasoline be wasted downhill in brake dust and heat. Do you know the official SCd figure for your car? Mine is 0.60m2, and at 80km/h my engine revs at 1300rpm, which is fine but couldn't be lower.
I might try overinflating the tyres but I will only do that on long trips.