My engine is stock, manual transmission. I bought the multigauge to ration my fuel, and routinely get over 30mpg, driving a mix of highway and city driving. Except for acceleration, I keep it between 5 and ~12 l/km (or whatever the measurement is on the gauge)... definitely keeping it to the left of center as often as possible helps. You can go 80mph and be doing around 10 l/km mileage, which translates pretty well.
One thing I noticed is that in Harrisburg, if you go the speed limit, you can go right through every traffic light like clockwork. It's funny watching people race to the next red light, when I'm just cruisin' along at 25mph, saving gas, and I don't have to stop... I know exactly when the light will turn green so I don't even slow down approaching the intersection. (Maybe just a little bit, heehee.)