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Most fuel-efficient luxury cars ranks the most fuel-efficient luxury cars according to annual fueling costs.
2006 Lexus RX 400h.
2006 Lexus RX 400h.Toyota / AP
/ Source: Forbes

Have you ever seen an SUV top a list of fuel-efficient vehicles? Get ready.

On Oct. 12, the U.S. Environmental Protection Agency (EPA) released its annual Fuel Economy Guide. This edition concerns 2006-model cars (the model year is already under way), and atop the list of fuel-efficient luxury cars sits an SUV: Toyota Motor's new Lexus RX 400h gas/electric hybrid.

Of course, the RX 400h is the most fuel-efficient luxury car not because it is a sport utility, but because it is a hybrid. The second-place winner, DaimlerChrysler's diesel version of the Mercedes-Benz E-Class sedan, the E320 CDI, is in that position not because it is a sedan or a Mercedes, but because it is a diesel car.

We examined the EPA's 2006-model data and distilled from it a list of the most fuel-efficient 2006-model cars, which you can find in the slide show that follows. The EPA's new rankings include many cars on the market, but not all, and feature some new arrivals that are just now going on sale or are about to go on sale, such as Audi's A3 compact and the Lincoln Zephyr sedan from Ford Motor.

We based our luxury-car rankings not on city/highway mileage but on annual fueling costs. This is because many cases come up where one car has better city mileage than another, but not better highway mileage. Annual fueling cost — data also provided by the EPA — is the best arbiter of the bottom line for fuel economy.

We also used only the EPA for mileage data. Some manufacturers provide their own fuel-economy figures, but these numbers are usually estimates prepared to give the press something to write about while the automakers wait for the EPA to provide official testing results. If a car was not ranked by the EPA, we did not include it in our calculations.

In our research, we only considered one entry per nameplate: the cheapest one to fuel. For example, BMW's 3 Series will generate different fueling costs depending on what kind of model you get: sedan or wagon, front-wheel or all-wheel drive, big engine or small engine.

Finally, perhaps the most tricky distinction to make in the process was what counts as a luxury car and what does not. For the most part, the difference came down to brand identities. We consider Honda Motor's Acura and General Motors' Cadillac subsidiaries luxury brands; we do not think of Buick or Hummer as such, even though the Brobdingnagian H2 rings up at a luxury-like $52,430.

Some non-luxury brands produce the occasional luxury car — for example, the excellent but anomalous $67,000 Volkswagen Phaeton sedan. Then there are some sports cars with premium prices, such as the Chevrolet Corvette and Dodge Viper SRT-10. We did not include these vehicles in our calculations because they are more about sportiness — way more — than about luxury. The Porsche Boxster convertible, however, achieves a sufficient balance of luxury and performance to merit a place on the list.