BMW hasn’t commented on a mysterious car shown in Chinese patent office renderings, but after speculation has already swirled, sources are now saying the sleek coupe is a plug-in hybrid design thesis with tech expected to find its way into future i-Series cars.

Previous conjecture had it that the car might be a fuel cell vehicle, but according to Autocar, the car (whose image we do not have to show) is actually a super advanced PHEV.

Autocar reports that carbon fiber and other lightweighting techniques along with extreme aerodynamics has enabled BMW’s prototype to be capable of 706 mpg on the combined fuel cycle.

This estimate is likely obtained using Europe’s more generous testing methods, and relying mainly on the battery, but this is serious efficiency any way one looks at it.

By comparison, the 2015 BMW i3 battery electric car with a range-extending engine is rated for 117 miles per gallon equivalent from the EPA.

BMW’s design thesis also is reasonably quick, reportedly reaching 62 mph in less than seven seconds, with a top speed of 112 mph.

Volkswagen XL1

Volkswagen XL1

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The power will come from a rear-mounted electric motor and a front-mounted turbocharged 1.0-liter twin cylinder engine, Autocar reports. Traditional battery options have been bypassed for an unnamed chemistry with “a newly developed chemical process for faster charging, greater energy density and a longer zero-emissions range,” said Autocar.

Curb weight for BMW’s plug-in prototype is estimated at under 2,535 pounds, and a sleek aerodynamic profile gives it a drag coefficient of 0.18 while using every trick in the book up to video cameras in place of rear view mirrors.

Sources also named the Volkswagen XL1 as the BMW’s main competition. Released in very limited production numbers, the XL1 diesel hybrid achieved 313 mpg (on the European Union’s combined cycle), sprinted to 62 mph in 11.9 seconds and weighed only 1,753 pounds.