Gas equivalent: Mercedes-Benz S550 (Base price: $95,000)
Savings on a hybrid: 2.8% Anyone spending more than $90,000 on a Mercedes S-Class car probably doesn't care that the S400 costs 2.8% less than its all-gas cousin the Mercedes S550, but buying the hybrid will help the environment as much as your pocketbook. That's because S400 gets an estimated 19 mpg/city, or nearly a third better than the 15 mpg/city that you can expect with an S550. (Both models get a projected 25 mpg on the highway.) And while the S400 lacks the S550's 429-horsepower V-8 engine, the hybrid does feature a V-6 engine and an electric motor that collectively generate a beefy 295 horsepower. Ecofriendly consumers might also consider the Mercedes S350, whose 240-horsepower clean-diesel engine offers 21 mpg/city and 31 mpg/highway -- the best efficiency of all for Mercedes ( DDAIF) S-Class models. The S350 does list for $650 more than the S400, but $2,000 less than the S550.