The limited-production Volkswagen XL1 ultra-economy two-seater gets astounding fuel efficiency from its two-cylinder diesel plug-in hybrid powertrain.

It's well over 100 miles per gallon if you use the European test cycle (which differs from U.S. tests).

But at a six-figure price and a production limited to a few hundred, it's not likely to have a huge impact on the car market.

DON'T MISS: Volkswagen XL1 138-MPG Diesel Plug-In Hybrid: Drive Report (Dec 2013)

Now comes news from German auto site Automobilwoche (via Auto Motor und Sport) that BMW is considering its own lightweight, ultra-efficient car.

Details are sketchy, but it is said to be a four-seater, with a structure largely made of carbon-fiber reinforced plastic (CFRP) just like that used in the BMW i3 and i8 electric cars.

The design is expected to achieve a blended 0.4 liters per 100 km on the NEDC European test cycle, equating to a stratospheric 588 miles per gallon.

Volkswagen XL1 (European model), New York City, Dec 2013

Volkswagen XL1 (European model), New York City, Dec 2013

It has far more interior volume than the VW, with four seats rather than the XL1's pair, and is said to have a drag coefficient of 0.18--lower even than the XL1's 0.19.

The tail is said to taper significantly, with a narrower rear track than in the front of the car--which is so far unnamed.

That very low drag is said to allow the vehicle to achieve a top speed of more than 112 miles per hour (180 km/h).

ALSO SEE: Ultra-Efficient VW XL1: Now An Urban Legend, Debunked By Snopes

BMW now has significant experience with carbon-fiber body structures: The BMW i3 and i8 use CFRP body shells mounted on aluminum rolling platforms that carry the running gear and crash structures.

Unlike those cars, however, the "0.4-Liter" BMW is entirely made of carbon fiber.

The weight is said to be 2650 pounds (1200 kg), against the BMW i3 battery electric version's weight of 2800 pounds in U.S. trim.

2014 BMW i3 electric cars on the assembly line in Leipzig, Germany

2014 BMW i3 electric cars on the assembly line in Leipzig, Germany

Most interesting is the two-cylinder gasoline engine, which is said to run solely as a range-extending generator--just as it does in the BMW i3 REx model.

The car itself is propelled by an electric motor on the rear axle, meaning the powertrain sounds quite similar to that of the i3 REx.

You could view the research vehicle, in fact, as a far more aerodynamic take on the i3 REx with an all-carbon structure.

2015 BMW i3 REx - Driven, Portland OR, April 2015

2015 BMW i3 REx - Driven, Portland OR, April 2015

But the experimental ultra-efficient BMW is likely to remain a research project, according to the German sources.

Producing it would be uneconomical at the moment, they say, so the company is not considering putting it into series production.

Would BMW do a small run of the car, perhaps at a price matching that of the XL1, just to underscore its advanced-technology capabilities--and prove that it could?

BMW i3 body shell

BMW i3 body shell

German automakers often move in lockstep, following each other's innovations in new body styles, electronic safety systems, and powertrain technologies.

So it's possible--although we think BMW would likely use the lessons it learns for future products that boost the fuel efficiency of its higher-volume plug-in vehicles.

Stay tuned on that one.


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