DETROIT – Don’t look for Volkswagen’s Passat BlueMotion concept at U.S. dealerships anytime soon. There currently are no plans to offer the car for sale here.

But executives say the concept is a key test case to see how far Americans are willing to downsize in order to maximize fuel efficiency.

Given relatively low gasoline prices in the U.S. “I wouldn’t have expected the clear desire from our customers for fuel efficiency,” Heinz-Jakob Neusser, in charge of the Volkswagen brand and head of VW Group powertrain development, tells reporters in a roundtable discussion at the North American International Auto Show.

“This is a clear downsizing (exercise),” Neusser says of the BlueMotion concept. “We’ll check to see how our customers react to the downsizing.”

One in three Passats sold in the U.S. now comes with a diesel engine, and VW says that percentage might have been higher in 2013 if not for limited availability.

Neusser says the automaker is moving toward localizing diesel production in North America, but expects it to take about two more years to get the engine into the mix at its year-old Silao, Mexico, plant.

The diesel shares the identical bore spacing with the gasoline engines made in Silao, so adding production would be easy, he says. But time is needed to line up parts sourcing with North American suppliers.

The BlueMotion Passat concept featured here this week incorporates a new version of VW’s 1.4L EA211 TSI 4-cyl. gasoline engine with direct injection and turbocharging to deliver an estimated highway fuel economy of 42 mpg (5.6 L/100 km).

That would be a best-in-class rating for a non-hybrid, gasoline-fueled midsize sedan, the automaker says. But it also would be the smallest engine offered in the midsize-car segment in the U.S.

Key is the addition of Active Cylinder Management technology previously seen on European-spec versions of the Volkswagen Polo and Golf. The engine dials down to 3- or 2-cyl. operation when maximum power is not needed, such as during steady-state cruising.

The car borrows from the Jetta Hybrid a version of VW’s dual-clutch DSG transmission that decouples during coasting to minimize friction. Stop/start technology also is incorporated.

The concept’s high-mileage engine is under consideration for the U.S.-market Golf, which will be built in Mexico with the new-generation launching this year, officials say.

Neusser says many of the concepts used for the BlueMotion Passat could filter into other vehicles, including lightweighting and aerodynamic techniques. Cylinder-deactivation and DCTs with the decoupling feature also could be applied to diesel engines, he adds, further improving fuel economy.

dzoia@wardsauto.com