Reviews July 20, 2010, 4:24PM EST

Review: 2010 Mercedes-Benz S400 Hybrid

With a list price of $88,825, the new S400 Hybrid is not only Mercedes' least expensive S-Class sedan, it is also its most fuel-efficient

By Thane Peterson

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Editor's Rating:

The Good: Price, improved mileage, all the comfort and luxury of an S-Class sedan

The Bad: Not as quick as the regular S550, pricey options

The Bottom Line: The cheapest, most fuel-efficient S-Class sedan on the market

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Up Front

The hybridization of high-end German luxury cars is under way. Daimler (DAI) kicked off the trend with the 2010 Mercedes S400, the gasoline-electric version of its top-of-the-line S-Class sedan. BMW (BMW:GR) has countered with its new, performance-oriented 2011 ActiveHybrid 7. In March, Audi (VOW:GR) showed off a hybrid A8 concept car at the Geneva Auto Show, and Porsche has promised a hybrid version of its new four-door Panamera next year.

Where does the Mercedes-Benz S400 fit in? Basically, it's a hybrid for people who don't want a hybrid, with new technology so seamlessly integrated into the design that you barely know it's there. Mercedes' first hybrid model, it's also the first production car to use lithium-ion batteries, which are small enough to fit into the engine compartment. As a result, the S400 has the same roomy interior and large trunk as a regular S-Class sedan. Trunk space is 16.4 cu. ft., more than in the Lexus LS 600h L (10.1 cu. ft.) and BMW ActiveHybrid 7 (14 cu. ft.).

The S400's other big innovation is its price: Most hybrids carry a premium over conventional gasoline-powered models, but the S400 starts at $88,825, $3,650 less than the least expensive V8-powered Mercedes S-Class sedan, the S550. That makes the S400 the least expensive S-Class sedan sold in the U.S., with a starting price far lower than those of the LS 600h L ($109,675) and the ActiveHybrid 7 ($103,175). (One concern about the Lexus: The LS 600h L has been the subject of two recent recalls, the most recent in early July for an engine valve glitch.)

The S400 is a so-called "mild" hybrid. Its power plant consists of a 3.5-liter, 275-horsepower V6 gasoline engine augmented by a 20-hp electric motor. Small as it is, the disc-shaped electric motor generates 118 lb.-ft. of torque that kicks in instantly when you punch the gas. The combined system generates 295 hp of power and an impressive 284 lb.-ft. of torque. The S400 isn't blazingly fast, but has more than adequate oomph for such a big vehicle.

The S400's electric motor is positioned between the gasoline engine and the seven-speed automatic transmission. The battery pack stores energy collected from a regenerative braking system. To further conserve energy, the gasoline engine shuts off at speeds of less than 9 mph, as well as during idling. At a stoplight, the air-conditioner compressor and power steering pump operate electrically.

The Environmental Protection Agency rates the S400 to get 19 miles per gallon in the city and 25 on the highway, for an average of 21 mpg. That's 27 percent better in city driving and 17 percent better overall than the rear-wheel-drive Mercedes S550, which is rated to get 15/23, for an average of 18. The S400 more or less matches the fuel economy rating of the LS 600h L (20/22 for an average of 21 mpg), and does a tad better than the ActiveHybrid 7 (17/26 for an average of 20 mpg).

However, I suspect many S400 drivers will achieve better than the rated mileage. I got 24 mpg in my test car over 512 miles of mixed driving, during which I made absolutely no effort to conserve fuel. Less lead-footed drivers will probably average 26 or 27 mpg.

S-Class sales are booming this year, at least by comparison with calamitous 2009. U.S. sales more than doubled in June vs. the same month in 2009, to 1,549, and were up 27.2 percent, to 6,405, during the first six months of this year. The S400 is a niche product, with only 524 sold in the U.S. during the first six months of the year, a mere 8.2 percent of total S-Class sales during the period. However, the S400 will seem a lot more attractive when gasoline prices rise again.