It's in the air. You can detect it in the popular press and celebrity media. There's a sense these days that in the sophisticated places-London, Hollywood, Ann Arbor-you're not really making the scene if you aren't green. In Europe, they've got the highly conspicuous Smart city car and a host of other micro-machines to prance around in to show that their hearts are in the right place. But here at home, there can be no better way to alert the world to the fact that you're green like money than with the new Toyota Prius gas-electric hybrid. Prius might almost rhyme with pious, but that's OK, brother, because you're religious in your zeal to conserve fuel and cut emissions, are you not? Or perhaps Your Righteous Dudeness would prefer to go green more subtly, facing down global warming with the sonorous turbo-diesel sounds of the extraordinarily abstemious Volkswagen Jetta TDI?

Fact is, home could be Graceland, and your other car might be a Hummer, but as far as the beautiful people are concerned, nothing says you're a dear old friend of Mother Earth more conspicuously than a car that can get 40 to 50 mpg or more. Any additional benefit from its thrift accruing to your selfish cheapskate side is a bonus-as is the underlying quality of these automobiles, which is considerable but more or less on top of the major environmental-status points they will win you. (Contrast this with the embarrassing, golf-cart-like GEM electric cars that some prominent Californians have let themselves be photographed in.)

We know all this, you see, because we got quite green ourselves recently, driving the Prius and the Jetta thousands of miles over the course of four months, commuting into, out of, and all around New York City, as well as on a series of road trips spanning the Middle Atlantic region of the Eastern Seaboard. The vehicles' mileage varied-and the hoped-for 50 mpg was attained only fleetingly-but social-impact-wise, we felt we stayed right on message, particularly with the Prius.

Let's face it. The Jetta diesel will never be as clean as a hybrid, because it's a diesel and because the hybrid spends a good part of its urban-running time in zero-emissions mode, driven by a mighty-mite electric motor fed by a hefty battery pack. That accounts for its stellar 60 mpg EPA city mileage figure, which many purchasers have mistaken for the mileage they might actually get around town, which, in our case, was far lower. Compared with the Prius, the Jetta is somewhat less stingy with a gallon of fuel and emits roughly twenty times more nitrous oxide, almost three times more hydrocarbons, and twice as much carbon monoxide, even though it's wildly clean by historic diesel standards. It helps to remember that the Prius, optimized to remain electric and hence emissions-free for a good portion of the EPA city-driving cycle, tests as one of the cleanest fossil-fueled cars in history. (Priuses certified for California and four Northeast states are cleaner yet.)The jury is still out on whether the diesel will survive in the U.S. market, though we're betting that the political muscle of diesel makers and other adherents will extend its life into a distant future. The question may be what form diesels take. There is nothing to suggest that they wouldn't offer similar benefits were they to be hybridized; indeed, the stated goal of the Partnership for a New Generation of Vehicles, the federally underwritten program to build an 80-mpg family car that cost billions and turned up nothing, was to facilitate production of just such a vehicle. The reason? Diesels use less of a more energy-dense fuel and therefore tend to be naturally low in carbon dioxide emissions-the bugaboo of the global warming debate-which is good. Emissions, especially oxides of nitrogen and particulate matter, remain a major concern, however, even if you can't see diesel soot with the naked eye as well as you used to.