Efficiency comes in many automotive forms: electric powertrains, continuously variable transmissions, smaller-displacement engines.

More aerodynamic designs help efficiency, too. With the 2019 A-Class, Mercedes-Benz says it's raised the aerodynamic bar.

Mercedes-Benz claims that the new 2019 A-Class sedan has the lowest aerodynamic drag of any production car in history.

2019 Mercedes-Benz A-Class L

2019 Mercedes-Benz A-Class L

Aerodynamic drag consists of two parts. The drag coefficient measures how slippery the design is. It combines with the frontal area of the car, or how much air it has to push out of the way as it runs down the road.

Automakers have made great progress in cheating the wind. It wasn't long ago that the best sedans undercut the 0.30 mark for drag coefficients.

Mercedes's claim is based on total drag, because the A-Class is a small car, smaller than the C-Class sedan sold in the U.S.

Its 0.22 drag coefficient ties with the latest BMW 5-Series, which is a larger car that punches a bigger hole in the air. The A-Class has a frontal area of 10.76 square feet (2.19 square meters) according to Mercedes.

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Mercedes originally introduced the A-Class as a tiny hatchback (think a Honda Fit from Mercedes) in Europe in 1997. Since then, the A-Class lineup has grown to include sedans, including the coupe-like CLA-Class.

While all types of cars have made great progress in fuel economy, aerodynamic drag is still one of the major factors that distinguish sedans from SUVs and even hatchbacks.

Sedans, with their tapered tails, set an aerodynamic standard on the road, one that SUVs and even hatchbacks, with their squared-off tailgates, can't match. That may be one reason Mercedes is focusing on sedans for the A-Class for now.

2019 Mercedes-Benz A-Class L

2019 Mercedes-Benz A-Class L

Modern crossovers SUVs based on cars can often approach the city gas mileage ratings of the sedans they're based on, but usually fall well short on highway mileage because of their aerodynamics.

That shortfall is a function not only of crossovers' higher ride height and taller profile—frontal area—it's also a result of higher drag from the crossover's squared-off tails.

Even among small cars that offer both sedan and hatchback body styles, the sedan almost always gets better fuel economy on the highway. As automakers continue to improve fuel economy, these differences look more pronounced, because mathematically, the higher numbers result in larger spreads when comparing different cars or city versus highway mileage.

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It’s not uncommon to see a 30-percent mileage improvement on the highway for sedans. Similar hatchbacks or SUVs with the same engine and same basic underpinnings get only a few mpg better on the highway than they do in the city.

Of course, aerodynamics don't benefit only gas cars. Lower drag gives electric cars longer range.

2019 Mercedes-Benz A-Class L

2019 Mercedes-Benz A-Class L

The 2019 Mercedes-Benz A-Class is also expected give rise to a new coupe-like CLA-Class sedan, as well as a hatchback outside the U.S.

When the CLA-Class debuted in 2013, it claimed the lowest drag at the time, with a coefficient of 0.23. Now that other cars such as the Audi A4 can match that, look for the upcoming CLA-Class to be even sleeker.

Mercedes revealed the 2019 A-Class at the Beijing Auto Show in April. It goes on sale late this year in the U.S.