Mine does edge down, but nor very much really. I think it's the law of averages. It's easy to increase the average early into a tank because of the law of averages. Halfway through a tank it takes a quarter of mile on all electric to move the average a tenth or two but early on it can jump as much as half a gallon over the same distance. It seems the same distance accelerating or on all ICE propulsion will drop it about twice as much. I do see it slipping as the tank goes by but I am fanatical about my mileage and focus on improvement almost every driving instance. The reward - I just did 63.0 mpg actual my last tank (400 plus miles).
