Pretty sure there is very little diference mpg between A4 and A6 with like for like engines and drivetrain - I have a 2004 A4 1.9tdi 130bhp - most mileage is motorway and on a steady run Bham to Newbury M42 M40 A34 I can usually expect to get on or near to 50mpg ..with a mix of 60mph on A34 80mph on M40 and 60 mph on M42.
The other half will regulary get upto 55mpg driving just 25 miles from cold on M42 and M40.
Check out figures from the technical data on Autotrader and compare diference between A4 and A6 - of course if you start looking at adding Auto / quattro and then vouple that with a heavier car above figures would be a tad optimistic ...but for a 130bhp tdi driven sensibly 50mpg on a motorway would not be out of your reach.