2014 Audi A8 diesel engine brings high MPG to executive sedan

When it comes to executive-class sedans, you can bet that automakers are showing off their most luxurious interiors and very lateste technologies. Traditionally you could also bet on those full-sized cars sucking down gas by the barrelful.

That dynamic is changing, and quickly. Audi has just released a diesel-powered version of its A8 long-wheelbase sedan, giving the company a more efficient offering against hybrid models from Lexus, Mercedes-Benz and BMW.

On the highway, the A8 L TDI is the class’s new gas mileage king, getting 36 miles per gallon. (It sees 24 in stop-and-go traffic.) Compare that to the top-of-the-line Lexus hybrid, the LS 600h L, which manages only 19 city and 23 highway. Mercedes also sells a diesel version of its iconic S-Class; it gets 21 and 31.

The A8 TDI starts at $82,500, making it pricier than the six-cylinder, long-wheelbase A8 ($78,800), but less than the 4.0-liter eight cylinder ($87,600). If you still prefer a whopping big engine, open your wallet for the W-12 model ($135,900).

The thought of mating a shiny car like the A8 with a diesel engine may seem odd. But the VW Group’s turbocharged direct injection (TDI) engines, which use ultra-low-sulfur fuel, are found on vehicles ranging from the VW Passat to the Audi Q7 SUV, and the powerplant is much loved. Diesels tend to make converts of drivers. Find a TDI driver and it’s likely he or she will proselytize.

Audi is pushing diesel across its lines. By the end of the summer, TDI versions of the A6 and A7 sedans and the Q5 SUV will also be available. Perhaps diesel’s time in America has finally arrived.

While the A8’s 3.0-liter six-cylinder TDI has only 240 horsepower, it has 407 pound-feet of torque. The car doesn’t leap forward with ferocity - 60 miles per hour takes 6.4 seconds - but it pulls up hills like a rogue elephant. It is never very loud, and it never seems to strain.

For my test drive, I had a special route in mind: A 660-mile, round trip from New York to northeast Vermont. Starting with just under a full tank, I figured I would need to fill up only once.

The beginning of the trip was a wretched attempt to escape Manhattan in the late evening bumper-to-bumper traffic and a torrential rain. The car had a defective driver-side wiper blade, leaving streaks of smeared water directly in my line of vision. All that fancy technology, felled by a strip of bad rubber.

It was, however, an ideal time to try out the car’s adaptive cruise control with stop-and-go functionality. Set the system and the car will adjust its speed according to traffic, inching forward or coming to a full stop when necessary.

After all, a major aspect of these executive-class cars is their technology, and both the A8 and the BMW 7 Series will be facing down the totally redesigned 2014 model-year Mercedes S-Class. The S-Class’s new adaptive cruise control is semi-autonomous and will even keep the car centered in its lane on curves.

To be honest, I’m leery of all this self-driving technology, fearing that the car won’t in fact stop on its own, and that I’ll be left explaining to the traffic police that the rear-ending accident isn’t my fault, but the car’s.

So the first time the Audi surged toward the bumper of another car, I kept my foot hovered over the brake. I got closer and closer and ... it stopped. Over the next hour the function made the traffic more tolerable. Still, it tends to make you less aware of your surroundings, more expectant that somehow the car will step in and save you.

Finally I cleared the traffic and let the A8 diesel do what it does best: Eat up highway miles. The all-wheel-drive sedan is stable, comfortable and utterly confident on the open road, even in the rain. I engaged the massaging seat function and blasted the $6,300 Bang & Olufsen sound system.

The LED headlights lit my way gorgeously and I made the destination, East Burke, around 1 a.m., behind schedule because of the rain, but with just under half a tank of gas.

In more than 600 miles of mixed motoring, with an average speed of 43 mph, the car saw 28.5 mpg, almost exactly its EPA rating. Over 300 miles of freeway driving, I got just under 33 mpg.

The average buyer of an executive-class sedan won’t choose a car for its MPG rating. Technology, comfort and great looks will still matter most. Nonetheless, it’s hard to imagine why you wouldn’t choose the diesel engine. Drivers will give up very little - except that extra stop at the gas pump on long road trips.