German automakers have spent time and money on ad campaigns promoting clean diesels as the better way to improve fuel economy, so far with limited results.

But that could change when Audi brings a TDI diesel version of its best-selling A4 model to the U.S. So far, its only TDI offerings are the Q7 full-size SUV and the A3 hatchback, neither one a high-volume model for Audi in the States.

Starting what will likely be a lengthy campaign to convince luxury sports sedan buyers that diesel is sporty as well as economical, Audi released the results of an A4 TDI fuel efficiency test on the European combined cycle: a remarkable 53.5 miles per gallon.

It's worth noting that U.S. EPA mileage ratings are always lower than the European numbers, but we would still expect the A4 TDI to return figures in the thirties.

By comparison, Audi's A3 TDI compact hatchback--which also uses the 2.0-liter TDI engine--is rated at 30 mpg city, 42 mpg highway.

2010 Audi A3 TDI

2010 Audi A3 TDI

Audi said earlier this week that by 2015, it would offer a TDI clean-diesel version of every model it sells in the U.S. The A4 TDI for the U.S. will come with a four-cylinder turbodiesel when it is redesigned for the 2015 model year.

That 2.0-liter TDI produces a modest 136 horsepower, but a more substantial 236 foot-pounds of torque from 1,750 to 2,500 rpm. That should make it quick off the mark, although a 0-to-62-mph time of 9.5 seconds is a long way from the speed of its hot-rod S4.

The European version is fitted with a six-speed manual. We would expect the U.S. model to have an automatic, most likely Audi's six-speed direct-shift gearbox as used in the A3 TDI, which shifts more quickly and reduces power losses.

Tweaks to complement the diesel engine include higher gear ratios, different wheels, and aerodynamic aids along with a lower ride height.

Last year, Audi sold almost 5,200 clean diesels in the U.S. That represented just 6.3 percent of its total sales of 82,716. However, the TDI models accounted for 43% of Q7 sport-utility sales of 7,299 and 53% of A3 hatchback sales of 3,874.

[Audi via Motor Authority]