The isn't great on a racetrack. This thought occurs during my third lap when I realize — the throttle wide open, exiting a corner at 100mph — this may not be the best sports car out there. My next thought is "But this is a 4,993lbs SUV." And that's the point where you realize what we have here is something entirely new.

BMW calls the X6 a Sports Activity Coupe. That seems preposterous given both their constant assertion that their best selling SUVs, the and , be referred to as SAVs and the X6's four doors, which is 2 more than the unofficial definition of a coupe allows. The is based on that X5, its main difference, aside from the obvious low roof, swoopy bodywork and huge ass, is firm suspension and the addition of Dynamic Performance Control.

Here it is, the first prototype we've seen for the new 2009 BMW X3 on the streets (and above at the

It's the latter that makes the X6 so special. Basically the cleverest rear differential in the world, DPC apportions power to the rear wheels independently to improve agility, stability and traction at all times. This matters to you and me because it makes the X6 handle incredibly well and incredibly safely in any condition without cutting power or hitting the brakes to interfere.

Combine DPC with Adaptive Drive, which eliminates roll and dive, and two incredibly powerful engines and you have a car that performs better than even the wildest imagination allows.

The first indication of the X6's genre-creating genius came with a tearing sound from the back end. Getting on the power in the middle of a wet and foggy hairpin in the Appalachian Mountains that sound suddenly joined the twin-turbo inline-six's deep growl as the level of grip began to beggar belief. It's the sound of the rear differential working hard to keep the X6 accelerating rather than spinning. Its function is otherwise undetectable, except through the scarcely unbelievable speed with which it allowed me to attack that windy mountain road in the middle of a thunderstorm.

Eventually, the realities of physics do overcome the ability of BMW's engineers to bend them. Pushed to the limits of grip the X6 will oversteer, stepping out into a few degrees of controllable slide, but take things a step further and the end result is understeer. The aggression it takes to provoke is, however, unlikely to be encountered on anything this side of an icy road or racetrack.

Slowing down, later in the day, that performance becomes nothing but a memory as the realities of driving an X6 on the road set in. It's bigger than you think, dwarfing Ford Explorers and minivans, a full width lane barely containing its bulk. This is both good and bad. The size creates an incredibly spacious interior for its four occupants and plenty of their luggage, but it's hard to place on narrow streets and in tight corners. The svelte shape leaves plenty of room for six footers in the rear seats, but doesn't allow for rear visibility. BMW has compensated by speccing the biggest wing mirrors you'll find this side of a semi, but we don't recommend reversing without the aid of the on board camera. And while its incredibly fast and has ridiculous levels of grip, the steering is almost totally numb, so it takes time to develop faith in the chassis' ability.

For the US, the X6 will come with two engines, both have twin turbos. The xDrive35i comes with a 3.0-liter straight six while the xDrive50i come with a 4.4-liter V8. On paper, the V8 is the clear winner, delivering a 0-60 time of 5.4 seconds and a top speed of 155mph. In practice however, the inline-six is livelier, weighing down the X6's front end considerably less and not suffering from the bigger engine's turbo lag. At 6.5 seconds to 60mph and topping out at 149mph, it's far from slow as well.

The gearbox lets both engines down by being difficult to use. Override it manually with the paddles and the results are hit or miss. Sometimes you get the gear you want, sometimes you don't, making fully exploiting the X6's chassis disappointingly difficult.

Don't expect the X6 to be the last vehicle to use Dynamic Performance Control. Allegedly, it'll soon find its way into the X5 and maybe even into vehicles from the M division. Judging by what's it's capable of here, we can't wait to try it when it's not just serving to make a flawed vehicle surprisingly capable, but when it's being used to make an already good car even better.