A wolf in sheep's clothing? That tag has never been more appropriate in the automotive world. Imagine pulling up at a stoplight next to what looks exactly like an Audi A6. Sure, there's an air dam up front and the hint of a spoiler in the rear, and the 18-inch Pirellis seem a little oversized, but you've probably seen customized A6s with more add-ons. You might look more carefully when the engine revs, emitting a low rumble not unlike some Italian exotic. The light turns green, and even though you're screeching your tires, all you glimpse are the tailpipes of that Audi racing away from you as you mutter, "What the hell was that?" Welcome to the world of the Audi RS6.

A recent Road & Track article also indicated this is no ordinary Audi. It matched the car in a runoff with a Ferrari Enzo and 575 Maranello, a Lamborghini Murcielago, a Dodge Viper SRT-10, a Mercedes-Benz's SL55 AMG and E55 AMG, and a Porsche 911 Turbo. While the Audi finished last at just over 16 seconds in this race to 100 miles an hour and back to a full stop, it wasn't racing out of its league. Most of the contestants finished in the 14-to-15-second range. All but two of the competition—the E55 and Viper—cost at least $50,000 more. And five of the cars couldn't take the wife, three kids and luggage for a ride at the same time.

The car's interior is outfitted more like a sports car than a luxury sedan. The custom-designed seats are aimed at the hard-driving enthusiast, who seeks firmness and security when cornering hard. The five-speed Tiptronic transmission is shifted from paddles on the steering wheel, providing instantaneous shifting without having to remove your hand from the wheel. In short, Audi's Quattro GmbH high-performance department has done everything to transform a sporty family sedan into a street-legal racing machine.

The RS6 hit American shores this summer, showing impressive numbers: a 450-horsepower, 40-valve, twin-turbo V8 that clocks 0 to 60 in just a tick over four seconds. No top speed is given, but if you're on a two-lane road looking to pass a speed-limit-observing driver, jumping from 60 to about 110 seems to only take a heartbeat or two, and you barely get it into fourth gear. The handling, underpinned by Audi's all-wheel-drive Quattro system, glues the car to the road, even in hard cornering on the back roads of Vermont. The price tag is high for an Audi: $82,000 before add-ons like GPS. There's one extra that seems worth it—and since the car is pretty much custom ordered, why not?—a custom color that transforms the car from the sleek but standard flinty gray. However you order it, compare it with the $228,000 for the Ferrari Maranello, or the $134,000 for the Porsche, and you might have an easier time justifying the cost to your family.

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