CB radio? An excellent guess, but no. Diesel, the oilier cousin of gasoline, dominates the European auto market, where fuel prices hover around $7 per gallon. Diesel is about 25 percent to 40 percent more fuel-efficient than gasoline, with commensurate per-mile reductions in carbon. The German luxury automakers -- Mercedes-Benz, Audi, Porsche and BMW -- are masters of turbo-diesel technology and have long asserted that it is, on balance, superior to hybrid technology. Now a confluence of factors -- the availability of ultra-low sulfur diesel fuel in the U.S., the emergence of California-legal exhaust-scrubbing technologies, higher fuel economy standards and spiraling fuel costs -- has set the stage for diesel's triumphant return to the U.S. Only problem? No buyers. For many Americans, diesel is simply smut. Audi and BMW are determined to change those perceptions with big, shiny ad campaigns to promote diesel. In August, BMW sponsored the season premiere of AMC's 'Mad Men' and purchased a slew of cable and print ads to promote its branded diesel technology called 'Efficient Dynamics.' BMW also sponsored a splashy homepage takeover of the New York Times Web site and MSN.com with ads featuring spokesman Brian Unger. The TV and Web ad creative duties were handled by GSD&M Idea City of Austin, Texas. To provide a visual counterpoint to the notion of dirty diesel, the BMW ads are stark white and dazzlingly lighted like a surgical theater, with the professorial Unger -- also rather stark white, if you ask me -- standing under a ceiling of compact-fluorescent light bulbs. The message comes through loud and clear: Diesel is cleaner and more efficient. 'This is the big push,' Patrick McKenna, manager of marketing communications for BMW, told AdWeek. 'For us, it's about changing the perception that diesel is still that noisy and smelly (technology) many people remember from the '70s.' BMW offers two diesels in the market: the 335d and the X5 xDrive 35d. In June, Audi staged homepage takeovers of Huffingtonpost.com, Slate.com, Politico.com and other newsy progressive sites. Its diesel campaign (Venables Bell & Partners, San Francisco) memorably features rusty oil drums rolling down streets, seemingly being repatriated onto tankers. The ad imagery was based on a factoid derived from an estimate by the U.S. Environmental Protection Agency: According to Audi, if one-third of U.S. drivers drove clean diesels it would mean the U.S. could 'send back' 1.5 million barrels of imported oil per day. And yet there was little mistaking a satisfying undertow of nationalism in the ads, if not xenophobia, as they seem to push back against the malign effects of 'foreign' oil. Audi disavowed any nationalistic subtext. 'We looked at it as a political campaign,' says Scott Keogh, chief marketing officer for Audi USA. 'We wanted to make diesel a much bigger idea that people could rally around.' The initial stage of Audi's diesel campaign scored big. The company got more than 300,000 Facebook visitors in the first month and traffic to the brand's U.S. Web site jumped 267 percent, with a total of 120 million media impressions, according to Keogh. It's interesting that the Audi Web site mentions its diesel competitors and the tenor of the campaign is generally more about supporting diesel per se. 'We're OK with that,' Keogh says. 'Anything that says that diesel is the right thing to do we're in favor of. Later, in late fall 09, when the A3 TDI (a premium hatchback) comes out, then we'll get into `Why Audi?' ' All this leaves the other two high-end German carmakers on the sidelines, comfortably. Mercedes-Benz was the first company to offer a 50-state clean diesel two years ago (E320 BlueTec) and now sells three diesel models. The company has done some diesel-specific marketing but, according to Mercedes-Benz spokeswoman Donna Boland, 'The most effective way to convince someone that diesel is right for him or her is to get them behind the wheel.' Meantime, says Boland, 'The (diesel) advertising has a cumulative effect in creating positive interest in diesel and therefore more floor traffic.' The company that stands to benefit most from the German's diesel push is one least associated with diesel: the hallowed sports car maker Porsche. The diesel Cayenne SUV accounts for 60 percent of the company sales. And even though the car is 28 percent more fuel-efficient than the comparable gas model, in the U.S., 'There is not currently a business case for this car,' says Michael Leitners, a Porsche product engineer. 'We wish there were.' Maybe soon, with the help of Porsche's competitors, there will be. 