BMW X5 Mercedes-Benz ML55 AMG Comparison

Mercedes-AMG has had the high-dollar, uberSUV market to itself for quite a few years. But the $69,025 BMW X5 4.6is, a full-boogie sport sedan disguised as an SUV/SAV, is a shot clearly directed over Mercedes' bow. It places strong emphasis on sport, moderately less on utility. The X5 4.6is is the only BMW product in our broad test not to wear an official M badge; according to BMW spokesperson Rob Mitchell, a prerequisite for the logo is a high-revving engine and a manual transmission. Though the 4.6is technically has neither, make no mistake: The DNA is there.

X5 4.6is Interior

The 3.0L/225-hp straight-six and now-290-hp 4.4L engines are still available, but the X5's top dog is now the 4.6L/340-hp powerplant. Compression has been bumped up from 10.2:1 (4.4L) to 10.5:1. Bore and stroke have been increased, as has torque, to 350 lb-ft at 3700 rpm. The final-drive ratio has been lowered to 3.91:1. The result is strong acceleration from rest, cracking off 0-60-mph runs in 6.20 sec. Top speed is electronically limited to 150 mph with the standard 20-in. rolling stock, 143-mph with the optional 19-in. wheel/tire combo.

ML55 AMG Interior

High-quality materials and well-thought-out ergonomics make the interior an ideal place to spend long hours putting large amounts of pavement in the mirror. The cargo area, not exactly generous in either vehicle, is better accessed in the X5 due to its sliding shelf and the spare tire mounted under the floor. Lifted from the M power parts bin is a variable warning segment on the tach, which encourages lower rpm upon cold startup; a line of orange LEDs that wink out as the engine oil warms up.

X5 4.6is Engine

Heated rear seats get power-adjustable back rests as part of the 4.6is package. The shift programming has been revised from the 4.4i; the 4.6is' "normal" setting is equivalent to the Sport setting in the 4.4i, while the Sport setting in the 4.6is is even more enthusiastic. The Steptronic five-speed automatic now has the intuitive manual shift pattern as used in a race car's sequential transmission, where pulling the lever back grabs the next higher gear.

The 4.6is employs a speed-sensitive rack-and-pinion-steering system, differing from the engine-speed-sensitive setup used on the 3.0i and 4.4i variants. Road feel is excellent, with little of the tram-lining you'd expect from its massive 275/40 front tires and 315/35 W-rated rubber in the rear. Directional transitions are predicable, with good feedback transmitted through the steering wheel. An impressive array of electronic aids are present, including Dynamic Stability Control, Automatic Differential Brake, Dynamic Brake Control, and Hill Descent Control.

ML55 AMG Engine

Introduced in '99, the AWD ML55 is presented as a utility vehicle with a heavy dollop of sport. Unlike the X5 4.6is, this SUV is intended for light off-roading-it even maintains a low-range transfer case. Its SOHC V-8 and running gear are assembled at the AMG facilities at Affalterbach, Germany, then sent to Alabama for final assembly. This handbuilt 5.4L wonder is an all-aluminum design, replete with matched pistons and connecting rods, hand-balanced crankshaft, modular camshafts, and topped with three-valve-per-cylinder heads. A tweaked ECU and high-flow fuel injectors result in 342 hp at 5500 rpm, with torque coming in at 376 lb-ft at 2500 rpm. Performance? With a 0-60-mph time of 6.24, it's a whisker away from tying the X5. The ML55 pulls strongly, especially as speeds increase; from rest to 100 the AMG takes only 16.48 sec, while the X5 needs 17.23 sec. Stomp on the ML55's accelerator, the revs come up, and you feel like you're aiming a speeding locomotive.