With fuel-efficiency and emissions regulations tightening all over the world, carmakers continue to push the limits on how economical a production car can get.

The latest entrant in the high-mileage stakes: a future edition of Volkswagen's Up minicar that will return 79 miles per gallon using a two-cylinder diesel engine paired with a hybrid system using a lithium-ion battery pack.

If that sounds vaguely familiar, it's because it's the same powertrain that Volkswagen intends to use in its limited-production, high-tech XL1 two-seater.

That car, the third iteration of a decade-long Volkswagen project to produce a "1-Liter" car (using 1 liter of fuel per 100 kilometers, or about 235 miles per gallon), was unveiled two weeks ago at the Qatar Motor Show.

The upcoming Up minicar will be one size smaller than the Volkswagen Polo subcompact. While it will be fitted with various small four-cylinder gasoline and diesel engines, VW is now planning to fit the XL1 powertrain in a special version that achieves maximum mileage.

volkswagen up concept motorauthority 009

volkswagen up concept motorauthority 009

The Up is a steel-bodied four-seater, versus the XL1's carbon-fiber two-seat configuration, so its fuel efficiency will obviously be lower. And, as always, real-world gas mileage depends entirely on how the car is used.

But the tiny 0.8-liter two-cylinder turbodiesel will produce 47 horsepower from what is essentially half of a Volkswagen 1.6-liter TDI diesel four. In the XL1, it is paired to a 20-kilowatt electric motor powered by a lithium-ion battery pack (of unspecified capacity) and a seven-speed direct-shift gearbox (DSG).

For the XL1, VW quotes an all-electric range of up to 22 miles. That could be lower in the heavier, less aerodynamic Up.

The ultra-high-efficiency Up isn't likely to be sold in the U.S., however. It's unclear whether the Up will be sold here at all, as earlier rumors of the subcompact Polo's launch in the States have not yet proven to be true.

Before launching a tiny minicar here, Volkswagen would logically have to extend its line below the current compact Jetta sedan and Golf hatchback.

Still, an 80-mpg Up with a diesel twin hybrid-electric powertrain would bring some big technology to a very small car. And it only goes to show that innovation is hardly dead; it's just likely to be costly.