comparisons are difficult, tyre sizes/pressures affect mpg as does the weather, massively. the hard cold truth is that modern diesels are no longer the economical reliable beasts they used to be. and its all down to emissions.
if you want economy buy an old early naughties PD or even better a pre-PD direct injection diesel, you;ll even get the reliability we once had, although try finding one with less than 150k on the clock and with all the creature comforts that you are used to in your modern audi.
EGR valves, DPFs, etc. all impact on MPG. if you want your economy back get rid of them pain in the !Removed! optional extras ;)