Big luxury cars are great. If you haven't actually had the chance to drive one, go try that 5-year-old Mercedes-Benz S-Class in the dealership's used-car section the next time you're there to buy a new Camry. But big luxury cars often come with other big things — big weight, big engines and quite a thirst for fuel. This is starting to change, however, as more of these large luxury cars start to feature alternative-fuel powertrains like the long-wheelbase 2015 Audi A8 L TDI and its turbocharged diesel V-6 engine.
Related: Research the 2015 Audi A8
German automakers have been at the forefront of diesel technology in passenger cars with all of them offering some form of it, and the fuel economy gains are significant. A standard Audi A8 L with the supercharged 3.0-liter V-6 engine is rated at 19/29/22 mpg city/highway/combined. Not bad for such a big car. Move up to the turbocharged 4.0-liter V-8 and your mileage drops to 18/29/22 mpg, a nearly identical rating. Splurge for the slick W-12 engine and you'll get only 14/22/17 mpg. But tick the diesel box and your big, comfy A8 L could return 24/36/28 mpg. The combined city/highway rating is 27 percent higher than that of the base engine.
Those are some impressive numbers for a vehicle this big and heavy, but are they realistic in real-world driving? We loaded an A8 L TDI with three people and some weekend luggage for a fall-color tour of southern Ontario earlier this year.