Launched in 1999, the X5 is BMW's flagship mid-size SUV, or as BMW calls it, a SAV, or Sport Activity Vehicle. From the very beginning, BMW worked hard to make sure that sporty driving dynamics and a high level of responsiveness were programmed into the package, which added not only a great deal of utility, but with BMW's standard all-wheel drive system, a sure-footed nature in a variety of terrains. Add to that BMW's trademarked build quality and luxury, and you have a versatile vehicle that's well-placed at the top of its class. Three trims are available, starting with the xDrive35i, the xDrive50i, and the diesel-powered xDrive35d. A high-performance X5 M model is also available. Several engine updates have been made for 2011, starting with a revised 3.0-L inline 6-cylinder turbocharged engine in the xDrive35i that makes 300 horsepower, compared to 260 offered by the twin-turbo version in 2010. xDrive50i versions have been fitted with a retuned all-aluminum twin-turbo 4.4-L V8 with variable valve timing and direct injection. It features what BMW calls a "reverse flow" setup, with its twin turbochargers situated within the valley between the cylinders. It makes 400 horsepower and 450 lb-ft of torque. The xDrive35d returns unchanged, with 265 horsepower and 425 lb-ft of torque from its diesel engine. At the very top of the range is the intense X5 M, featuring 555 horsepower from a tuned version of the xDrive50i's 4.4L V8. Both the xDrive35i and the xDrive50i receive an all-new 8-speed automatic transmission for 2011, first introduced in the 5-series Grand Turismo. It is claimed to weigh no more than the 6-speed auto it replaces, but offers closer gear ratios for better acceleration and two taller overdrive ratios for lower RPMs while cruising. Brake energy regeneration has also been added to both of the new engines, which uncouples the alternator from the accessory drive system, allowing the battery to only be charged during coasting or braking. This, BMW says, results in up to 2 percent fuel economy boost. Both the xDrive35d and the X5 M retain 2010's 6-speed automatic as the only transmission option. Standard safety features include a myriad of airbags--driver's and passenger's front and side units as well as head airbags for the first- and second-row seats. In addition, BMW offers Dynamic Stability Control, Dynamic Traction Control, Dynamic Brake Control, Hill Descent Control and Dynamic Cruise Control. For 2011, Active Cruise Control and Lane Departure Warning systems have been added. There's also a rearview camera that can provide a 360-degree exterior view, a Heads- Up Display that projects onto the windshield in the driver's field of view and a Park Distance Control system that alerts the driver of obstacles behind the vehicle while parking. The xDrive35i is available in three trim levels, with the base model including automatic xenon adaptive headlamps, rain sensors, HD radio, fog lights, automatic climate control, cruise control, 18-inch wheels and an anti-theft system all as standard equipment. Premium adds 19-inch wheels, iPod integration, tinted glass, leather seats and the availability of a Convenience Package, Active Ventilated Seat Package, Cold Weather Package and Technology Package. The top of the line Sport Activity adds 20-inch wheels, sport seats, performance tires, BMW's Adaptive Drive System and special darkened trim.