It depends entirely on the type of driving you do and how the car is driven. My tank average is around 20-22MPG if i drive only around town, which is always in traffic, speeds bumps everywhere, short journeys etc. I know my car is running faultlessly too, no PCV issues, boost leaks or anything like that - all down to type of journey and manner of driving
On the motorways it'll easily average high 30's and i once managed 45mpg being incredibly careful on a 60 mile round trip, door to door too, not just reset once up to motorway speeds.
Check the obvious things first, binding brakes, tyre pressures etc. Adjusting your driving style would have the biggest influence.