For that kind of mpg the Audi TDI has to come with the 2.0L TDI engine. Quattro weighs the car down so look at the kind of mpg that the current Audi A3 TDI gets and then add a few hundred pounds. 35 mpg seems doable but I wouldn't expect much over 40 mpg average.
I want the 3.0L engine but the 2.0L engine is honestly the best choice. BMW and MB have the 3.0L engine and sales are not the success that the VW and Audi TDI are. The other cars should have come with their 4 cylinder as MB's E320 2.2L Bluetec engine makes 204hp and 369 lb-ft. That's as much torque as a gas V8 and would have no problem moving the cars. Instead, the only have the E350 V6 diesel engine with 210hp and 400 lb-ft. The extra cost, weight, and fuel economy decrease just isn't worth it.
So Audi, you better have a 4 cylinder TDI or repeat BMW and MB's mistakes.my two cents Or at least have the 6 cylinder option...the engine/drivetrain is already used in the Audi Q7 TDI and Touareg TDI.