Launched in 1999, the X5 is BMW's flagship mid-size SUV, or as BMW calls it, an SAV, for 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 X5. This means the X5 features not only a great deal of utility, but with BMW's all-wheel drive system, a sure-footed nature in a variety of terrains. Add to that BMW's trademarked build quality and luxury, and the X5 is a versatile vehicle that belongs at the top of its class. Four trims are available, starting with the rear-wheel-drive sDrive35i and topping out with the all-wheel-drive xDrive50i. The 3.0L inline 6-cylinder turbocharged engine in the sDrive35i and all-wheel-drive xDrive35i makes 300 horsepower. The xDrive50i is fitted with an all-aluminum twin-turbo 4.4L 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. This setup is good for 445 horsepower and 450 foot-pounds of torque. Powering the xDrive35d is a 255-horsepower 3.0L inline-6 diesel engine. The Variable TwinPower Turbo engine also produces 425 foot-pounds of torque and delivers an EPA-estimated 26 mpg on the highway. The sDrive35i, xDrive35i and the xDrive50i use an 8-speed automatic transmission, first introduced in the 5-series Grand Turismo. Brake energy regeneration is also used on both gasoline 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. Standard safety features include several 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. Active Cruise Control and Lane Departure Warning systems are also available. 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 Luxury Line includes 19-inch alloy wheels, chrome window surrounds and a sport leather steering wheel. The xLine features separate 19-inch wheels, satin aluminum window surrounds and a silver matte grille surround. Finally, the M Sport includes an M Sport steering wheel, a sport automatic transmission, sport seats and M-badged foot rests and door sills.