Big cars with adequate gas mileage are hard to come by, but one of the best options at the moment is diesel power.

At its best, the BMW 7-Series is a bit of a gas-guzzler. With a turbocharged, 3.0-liter inline six, the best it can attain is 29 mpg highway.

Throw in a diesel engine though and it should improve--so goes the theory behind the latest addition to the range, the 740Ld xDrive.

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The name is a bit of a mouthful--the 'L' stands for BMW's long-wheelbase chassis, and 'xDrive' is BMW-speak for an all-wheel drive transmission.

But the 'd' is the important bit, since it refers to the car's 3.0-liter, twin-turbocharged inline-six cylinder diesel engine. This develops 255 horsepower at 4,000 rpm and a useful 413 lb-ft of torque between 1,500 and 3,000 rpm.

All that grunt is filtered through an eight-speed automatic transmission and the aforementioned all-wheel drive system; 0-60 mph acceleration takes just 6.1 seconds.

The biggest numbers should be the car's economy, but there's no official EPA figure just yet.

BMW points out that in other models equipped with the same engine, fuel efficiency has risen by 25 to 30 percent. With the BMW 535d for example, using the same six-cylinder diesel, combined mileage is 6 mpg better than the 24 mpg combined 535i.

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If that trend continues, we can expect figures of  around 24 mpg city, 36 highway and 27-28 mpg combined from the big Bimmer. Driving Dynamics Control with an Eco Pro mode and standard start-stop should help drivers minimize their fuel use.

The 740Ld should be clean, too--its exhaust gas treatment system is specifically designed to comply with ULEV II emission standards. A NOx storage catalyst, diesel particulate Filter and a Selective Catalytic Reduction (SCR) system all ensure harmful emissions are kept to a minimum.

All this technology (and all this car) doesn't come cheap--pricing starts at $83,425 including the mandatory $925 destination charge--but owners will at least save on fuel.

You'll get your first glimpse of the car at next month's Chicago Auto Show, while deliveries begin Spring 2014.


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