Coming to America: European Gas Sippers

European luxury-car makers are going to give U.S. consumers some tastes of the forbidden fruit they have been keeping to themselves. The goodies in question are some of the more fuel-efficient variations of popular models sold in the U.S., and perhaps some of the small cars and crossover wagons that have until now been offered only in Europe and other markets where fuel prices are high. Mercedes Benz, Audi and BMW all are planning to launch in the U.S. new cars—or new engines for existing models—that offer more advanced fuel-saving technology.

Travelers to Europe know that the big German luxury brands have been holding out on the U.S. for years. Consider BMW AG. The most efficient 3 Series model in the U.S. is the BMW 335D, a diesel model rated at 27 miles per gallon. BMW only started offering a 3 Series diesel in the U.S. about three years ago.

But the one Americans get isn't the best BMW can do when it comes to fuel-saving technology on the 3 Series line. For that, you still need to go to Europe, where BMW offers a model called the 320d Efficient Dynamics sedan (or saloon, in Euro-terms). This comes with a variety of petrol-saving features, such as an automatic start-stop function that shuts off the engine at stop lights, low-friction tires, and a system that uses braking energy to recharge the battery. These allow it to put out just 109 grams of carbon dioxide per kilometer under Europe's mileage-measuring system, which translates to roughly 57 miles per U.S. gallon. The European 335D, roughly comparable to the U.S. model, is rated at about 35 miles per gallon.

It's easy to hop on the Internet and find mileage ratings for cars sold in Europe. But comparing European and U.S. mileage and carbon-dioxide ratings is a difficult, apples-to-oranges problem, industry and government officials say.

Europe uses a different—some industry officials say overly optimistic—set of calculations to estimate fuel efficiency (usually expressed as liters per 100 kilometers or grams of carbon dioxide per kilometer, instead of miles per gallon). European regulators also give more mileage credit for technology such as stop-start systems than the U.S. does. Counting grams of carbon dioxide emitted per kilometer is a different way of measuring gasoline consumption since the more gasoline or diesel fuel a vehicle consumes, the more carbon dioxide it will emit. U.S. regulators are starting to compare vehicles based on carbon-dioxide emissions.

Europe uses different testing methods to estimate fuel consumption in city and highway driving. And U.S. regulators adjust mileage test results down in an effort to match the values advertised to consumers more closely to the likely mileage they'll get in real-world driving.

Still, it's still clear that U.S. consumers haven't gotten the most advanced fuel-efficiency technology the luxury-auto industry could offer. Among the reasons: U.S. clean-air regulations require expensive exhaust-scrubbing technology on diesel engines, and most U.S. luxury-car buyers measure prestige in terms of horsepower and performance, not fuel efficiency. Car makers worried customers wouldn't pay up for expensive technology designed to cut fuel consumption. Even now, fuel costs aren't a real financial challenge for wealthy households.

But who feels smart spending $60 or $70 filling up on premium gasoline? Moreover, U.S. regulators have signaled they plan to demand big, additional gains in fuel economy when new federal mileage targets are issued this fall for the 2017-2025 period.

Facing these pressures, European luxury-car brands are moving to offer U.S. consumers more of the high-mileage models and engines once kept back in their home markets. This will delight some Americans who have lusted after high-performance diesels and nimble four-cylinder models they once had to fly across the ocean to drive. Others will take a more practical view: Fewer top-of-the-line Mercedes models will come with a gas-guzzler tax added to the price.

Leading the parade of more Europeanized European cars is the 2012 Mercedes S Class diesel—the first diesel S Class the brand has offered in the U.S. since 1996. The all-wheel-drive, 3.0 liter, V6, S Class diesel is expected to average 20 miles to the gallon in the city, and 31 on the highway—a substantial improvement from the current S550 4Matic, which is rated at 14 city, 21 highway. The S-diesel's fuel efficiency is also better than the current S Class hybrid.

Mercedes is going for better mileage across its lineup. The brand already had added stop-start systems to its ultra-high-performance S63 and CL63 AMG models, and substituted a smaller V8 engine that uses direct fuel injection and turbo charging to boost power. The result: 563 horsepower and an Environmental Protection Agency mileage rating of 15 city, 22 highway—and no guzzler tax.

Mercedes also plans to start offering a four-cylinder version of the C Class for 2012, and replace its current 5.5-liter V8 in many models with a smaller, more efficient, 4.6-liter eight-cylinder engine.

Audi, the luxury unit of Volkswagen AG, has been relatively cautious about bringing its European diesel engines to the U.S. But for 2012, the brand plans to expand its Euro-diesel offerings in the U.S. to the top of the line A8 sedan, the A6 and the Q5 crossover. The company's best-selling model in the U.S., the A4, might be offered with a diesel at some point, but Audi isn't saying yet.

Audi is still withholding some of its most fuel-efficient vehicles, including the subcompact A1. And the company is being coy about when it will launch a U.S. version of its compact Q3 crossover, officially unveiled at the Shanghai Motor Show last month. That vehicle saves fuel with a stop-start system, but an Audi spokesman says the company isn't convinced U.S. customers will embrace the feature. "Americans think something's wrong with the car," he said.

Other car makers are starting to offer vehicles with this technology. General Motors Co.'s Buick brand plans to offer a stop-start system branded eAssist on its 2012 LaCrosse and Regal models. As for BMW, the company is making plans to offer at some point a U.S. version of its compact X1 crossover—a vehicle that is a size down from the X3 model that is currently the most-efficient crossover BMW sells in the U.S.

Whether there's a mass market in the U.S. for a BMW crossover that is roughly three inches shorter than a Ford Focus sedan is no sure thing, even if it does average about 45 miles per U.S. gallon in the European mileage tests. A BMW spokesman says the American debut of the X1 has been delayed "due to demand in other parts of the world."