The Least Fuel Efficient Luxury Cars

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German luxury cars are legendary for their stout construction and powerful engines that enable them to serenely cruise at triple-digit speeds.

As our ranking of the least fuel-efficient luxury cars shows, they are also proficient at guzzling gas.

The entire list, with the exception of a lone American sedan — the Cadillac STS-V — is composed of cars from Audi, BMW, and Mercedes-Benz, with the last two brands dominating the ranking.

The reason German cars dominate the ranking of least fuel-efficient luxury vehicles is because Audi, BMW, and Mercedes-Benz produce some of the fastest, most powerful cars in the world. The vehicles on our list are souped-up versions of standard models — specially equipped, limited-editions that incorporate racing technology, and are made for drivers who crave velocity above all else. Each company has a special moniker included in the name of these rare vehicles to help set them apart: Audi uses the designation "S," BMW uses an "M," and Mercedes-Benz applies the letters "AMG."

Weight and engine design are two major factors that significantly affect fuel economy, and most of the models on our list return poor mileage because they are heavy and have huge engines that crank out a lot of power. For example, the BMW M5, tied for first on the list, is 500 pounds heavier than the 528i sedan on which it's based. This is because the M5's V10 engine is much larger and more powerful than the 528i's six-cylinder engine; plus the M5 is laden with added technology to keep it stable under hard acceleration and at high speeds — not to mention more luxury amenities to help justify the $40,000 price difference between the two sedans.

As a result, the 500-horsepower M5 returns 13 mpg in combined city/highway driving, while the 528i, which puts out less than half the power of the M5, gets 21 mpg.

Luxury automakers are careful to point out that their low-volume, low-mpg cars exist within expansive model lineups. The companies structure their product portfolios to please diverse tastes, including some shoppers willing to drop nearly $200,000, as is the case with the Mercedes-Benz CL65 AMG, one of the models tied for least fuel-efficient luxury car, with average fuel economy of 13 mpg.

"We sell a lot of the M cars to purists: real automotive enthusiasts," says Melissa Steffy, general manager of Herb Chambers BMW in Boston. She says that buyers of BMW's highest-performance cars — those that have an "M" in their names — really value their extreme power and handling potential and don't care as much about fuel economy.

They're also not just spending a lot for exclusivity. "It's not like driving a Ferrari or an Aston Martin, which is really in your face," Steffy says. "You have to be a real enthusiast to know what an M car is. It sells to people who've done a lot of reconnaissance before they come in the door, people who have done a lot of research."

Compared to more mainstream models in their lineups, manufacturers don't sell many of the vehicles on our list. Audi sells about 6,000 S models a year, which is about 10 percent of total sales volume. BMW sees between 7,000 and 15,000 M purchases a year, which is about 2 percent to 5 percent of the company's total sales. Mercedes sells around 10,000 high-performance AMG cars each year, equal to roughly four percent of U.S. sales.

"Ultra high-performance models are often a small piece of the whole puzzle," says Cadillac spokesman David Caldwell. "The [Cadillac] V-Series represents a pretty exclusive club, and that's how it is designed to be."

Members of that exclusive club traditionally haven't placed a high value on fuel economy, but increasing social and political pressure could prompt manufacturers to wring more mileage from even high-performance luxury cars, says Michael Robinet, vice president of global vehicle forecasts for automotive market analyst CSM Worldwide. "The focus will be on making them lighter so they get better fuel economy," he says. The good news for enthusiasts only concerned about speed and power is that reducing weight not only improves mileage, but can also increase performance without needing to boost engine power.

At the same time, luxury brands are investing in new technology, some of which may provide heady power without penalties in fuel consumption. For example, BMW and Porsche plan to introduce hybrid models in the U.S. starting next year. And at the 2007 Frankfurt auto show, Mercedes showed multiple alterative-fuel vehicles, including gasoline and diesel hybrids.

"Our current plans call for bringing in hybrids in our S-Class and ML model lines beginning next year, and building out from there," says Donna Boland, spokesperson for Mercedes-Benz USA. Both will sell alongside fuel-efficient diesel-powered models. "It's not an either/or proposition. Given different driving demands and different customer preferences, we plan to offer both," she says.

But for right now, fast luxury cars still demand lots of gasoline. To identify them, we used the city and highway mpg ratings published in the EPA's 2008 Fuel Economy Guide and calculated combined fuel economy according to a formula provided by the EPA, carrying the calculations out to two decimal points. In cases where multiple transmissions and body styles are available, we averaged their EPA ratings. This ranking includes only mainstream luxury brands, excluding models from companies like Ferrari and Lamborghini that sell a relatively small number of vehicles each year and are in a niche of their own when it comes to price, performance, and fuel economy.

We aimed to identify just the top 10 luxury cars with the worst fuel economy, but because these brands have so many cars that return dismal mileage, we ended up with 21 models because of ties.

Only about 3 mpg separates all the models grouped in our ranking. All are stylish and relatively rare, and can deliver you to a destination at speeds that will make it hard for you to be late. Unless you have to stop for gas, that is.

Copyright © 2008 Forbes.com LLC