Audi's A8 shows the diesel difference

Big luxury cars mean big gas consumption, but that's starting to change -- and Audi's A8 is a good example.

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 miles per gallon 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.

The 300-plus-mile route took us from Ann Arbor, Mich., through Detroit and over the Detroit River into Ontario, where a combination of highway and country roads kept the A8 at a reasonable speed. We stopped in various small towns and at roadside attractions to sample the end-of-season fish fry and a couple of the region's wineries.

The A8 L is a remarkable touring machine, packed with technology and trimmed to an impressive level with wood, leather and faux suede. Front seat or rear seat, there isn't a bad place to sit, especially with the Luxury Package that my $98,575 test car featured. The panoramic sunroof lets plenty of light shine onto the diamond-stitched Valcona leather interior, and the open-pore wood trim looks dynamite. There's massive legroom for backseat passengers thanks to the extended wheelbase, and those backseats are heated, which was useful in chilly Ontario

The real story here is under the hood, with the turbocharged 3.0-liter diesel V-6 making a relatively tame 240 horsepower but a massive 428 pounds-feet of torque. This is characteristic of most turbo-diesels, which don't generally rev very high but make a significant amount of grunt at lower rpms. As such, the A8 L TDI can go from zero to 60 mph in a respectable 5.9 seconds, according to our friends at "MotorWeek," passing the quarter-mile traps in 14.4 seconds. Not bad for a car that weighs more than 4,500 pounds and is rated at 28 mpg combined, a number it easily surpassed on my trip.

In my experience, diesels almost always get better than their EPA figures, and the A8 L TDI was no exception. During my 328.3-mile voyage, it consumed just 9.63 gallons of diesel fuel. That translates into a combined 34.1 mpg, which is considerably more than the EPA's rating, and closer to the 36 mpg highway figure. That's extraordinary for this big Audi, and even bests some compact so-called "economy" cars.

Right now, gasoline prices have fallen dramatically as fuel economy of the overall fleet improves. Diesel prices have not come down, however, and still hover around $3.30 per gallon mark, as gasoline falls towards $2 a gallon. This makes the extra cost for a diesel engine (a $3,700 premium over an A8 L 3.0T) plus the extra cost for fuel a difficult sell right now. But given the cyclical nature of gas prices, having engine options like these makes sense over the longer term.