LAS VEGAS – Don’t be fooled by the seriously suave demeanor of the low-slung ’14 Audi RS 7, the high-performance variant of the 5-door A7 liftback that reached U.S. shores in spring 2011.

Sure, the car cruises the interstate with the near silence of a zero-emission electric vehicle, subjecting occupants to only the slightest road and wind noise.

This is when Audi’s sophisticated cylinder-deactivation system is at its best, depowering four cylinders so seamlessly that an unsuspecting driver might think all eight combustion chambers have gone into sleep mode, starved of fuel while the car sails along on sheer momentum.

It’s at this time the driver may think the 560 horses corralled inside the compact new TFSI 4.0L twin-turbo V-8 have gone missing from the ranch. But a firm kick of the accelerator finds them bolting in unison up the hillside, stirring up a furiously noisy cloud of dust as the rest of the farm animals scurry for cover.

This bucking bronco requires a steady hand on the reins, because he will run hard – well past the stated 174 mph (280 km/h) top speed, according to some independent tests – and is perfectly content in a full-on sprint. From a standstill, the RS 7 can giddyup to 60 mph (97 km/h) in 3.7 seconds, although some media outlets have managed it in 3.4 seconds.

Such is the reality of the curiously bipolar RS 7, a 4-passenger car with exotic speed that achieves real-world fuel economy of 23.4 mpg (10 L/100 km) during a 245-mile (394-km) jaunt through the desert here. It seems this unbridled muscle car is a pussycat at heart.

Not that the A7 (starting price of $64,500) needed a powertrain overhaul. Its “base” 310-hp 3.0L supercharged V-6 is no slouch, having earned Ward’s 10 Best Engines trophies the past five years.

The fuel-efficient play in the A7 is the new 3.0L TDI diesel V-6, offered for the first time this year in the 5-door sportback (starting at $66,900) and boasting a 38-mpg (6.2 L/100 km) highway fuel-economy rating.

Stepping up to the $80,200 S7 model finds a detuned version of the TFSI 4.0L V-8, rated at a mere 420 hp.

One rung upward sits the high-revving $104,900 RS 7 thoroughbred, which is now on sale.