i've just managed to get 36.2 mpg (tank average) out of my 1.8t sport 180bhp doing a roughly 50/50 mixed city/open A road run from Sheffield to Worksop of 18 miles each way, i did mostly careful driving but did give it the odd boot and air con on most of the time. I've worked this out doing 1 gallon = 4.54 litres x X litres used to cover X miles as i don't have a fuel computer.
considering the 1.8 turbo petrol engine, power, weight of the car it's pretty damn good