I think I finally solved the mystery, why my fuel computer showed better values when driving faster.
Leaving home on a cold morning, during the first 5 minutes, the mpg readings were lower than I was used to at 40mph.
When the engine temperature got hotter the readings were back to "normal"
That's when I realized that my mechanical fuel pump heats the fuel, because there is no adequate insulation between the pump and the engine.
And as the fuel boils very easily, it starts creating bubbles in the fuel line.
And the fuel sensor has an optical eye that counts the revolutions of the circulating ball.
And this eye counts also the bubbles coming from the pump.
So when driving faster there is more fuel flow and the heat has less time to boil the fuel and mpg readings look better.
So, I placed the sensor before the pump in the fuel line and quess what was the result
Now my fuel computer encourages me to drive slower as the readings go very much with the load.
Even driving behind a truck makes a difference!
Now I don't have to prefer the highways any more. I can choose the easy country roads and relax and enjoy driving
