Smart, with its line of electric or gasoline two-seaters, has the best gas mileage average among car manufacturers, according to a new Brighter Planet study of 2011 cars and trucks.

Throughout its lineup, Smart averages 61 mpg. Mini (30.0 mpg) is second across its production line. Honda (27.4 mpg) third is and the top manufacturer with a full line of cars, trucks and SUVs.

Suzuki (25.5) and Nissan (25.3) round out the top five, according to the study that detailed Brighter Planet’s ranking of 39 automobile manufacturers included in the 2011 EPA Fuel Economy Guide based on miles-per-gallon fuel consumption or equivalents of all models in each company’s portfolio.

Bentley has the dubious honors in the study, the lowest manufacturer-wide gas mileage average, 14.0. Rolls-Royce and Lamborghini are only marginally better.

Brighter Planet devised its survey by averaging the mpg fuel consumption of all models in each company’s line of 2011 vehicles.

“Looking at the entire range of models rather than a single car gives a good indication of a manufacturer’s overall commitment to energy efficiency,” said Ian Hough, Brighter Planet’s scientist responsible for data and calculations. “It shows which companies are improving fuel economy across their full line of vehicles.”

The most energy-efficient vehicles in the current 2011 EPA Fuel Economy Guide are the Nissan Leaf with a gasoline equivalent of 99 mpg; the Smart Fortwo Electric Drive Cabriolet and Coupe, both at a gasoline equivalent of 87 mpg; and the plug-in hybrid Chevy Volt at a combined gasoline and electricity value of 60 mpg. The best non-plug-in car is the Toyota Prius at 50 mpg.

“There’s a clear split in the automobile industry between companies like Smart and Mini that focus on a limited line of small, efficient cars and companies like Chevy, Toyota, and Nissan that have some great vehicles but are dragged down by all their trucks and SUVs,” said Hough.

“Chevy’s Volt gets good mileage, but when you look at Chevrolet’s entire production line, it ranks in the bottom ten. Honda is the real winner among the more conventional manufacturers. Even though one-third of their line is trucks and SUVs, their overall fuel economy is just 9 percent behind Mini.”

Here’s the list of manufacturer averages for 2011 models:

Smart, 61.0; Mini, 30.0; Honda, 27.4; Suzuki, 25.5; Nissan, 25.3; Mercury, 25.2; Hyundai, 25.2; Kia, 25.1; Volkswagen, 24.6; Lincoln, 24.3;

Mazda, 24.2; Toyota, 23.8; Saab, 23.6; Lexus, 23.4; Mitsubishi, 23.4; Volvo, 22.3; Chrysler, 22.3; Audi, 22.2; Subaru, 21.9; Buick, 21.5;

Ford, 21.2; Porsche, 21.1; Infiniti, 20.5; Jeep, 20.5; BMW, 20.4; Mahindra, 20.4; Acura, 20.2; Mercedes-Benz, 20.0; Dodge, 19.9; Chevrolet, 19.6;

Jaguar, 18.8; Cadillac, 18.2; GMC,17.0; Maserati,15.7; Land Rover,15.7; Aston Martin,15.0; Lamborghini, 14.7; Rolls-Royce, 14.6; Bentley, 14.0.

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