With the seven-passenger GL320 BlueTec, Mercedes-Benz is using blue to go green. The improved combustion of the BlueTec engine efficiently uses ultralow- sulfur diesel—in addition to a technologically advanced exhaust purification process that employs a urea-based solution called AdBlue—to reduce emissions. Nitrogen oxide—a harmful byproduct that is higher in diesel fuel than gasoline—is broken down into ammonia when AdBlue is injected into the exhaust flow and is released as harmless nitrogen and water vapor. On average, the 8.5-gallon AdBlue tank requires service every 10,000 miles. The GL320 BlueTec’s V-6 engine achieves an EPA estimated 23 mpg on the highway, but turbo diesel engine in the GL320 BlueTEC is not just quiet, clean, and fuel-efficient—with 398 ft-lbs, it surpasses the torque output from the GL550’s 5.5-liter V-8. Pros: Great mileage and tons of torque. Cons: Its large size may make it difficult to navigate through congested freeways.