Road Test: BMW X5 4.6is vs. Mercedes-Benz ML55 AMG
Do you value sport or utility?
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.
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.
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.
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.
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.
Anyone familiar with current Mercedes-Benz products will feel right at home in the ML55. The suspension has been lowered 0.3 in. from standard MLs and is softer than the BMW, not hindering the ML55's ability to safely cross unpaved terra firma. The independent front suspension utilizes torsion bars and shock absorbers, while the rear has a double-wishbone independent setup. Each corner uses 285/50R18 tires, and the relatively tall sidewall helps soften rough surface impacts. But it also compromises handling. Slalom tests cast each vehicle in a different light. The sport-heavy X5 carved up the cones at 63.13 mph, while the ML55 danced through at only 59.99 mph. "Only" is a relative term, as the ML55 will still outpace 99 percent of all SUVs. While the ML55 used 130 ft to stop from 60 mph, the X5 needed only 117.
One disappointment with the M-B was the high level of wind noise seeping into the cabin at freeway speeds. For $66,545, we expected a quieter interior. The non-telescopic steering wheel is another. A third is the large spare tire mounted in the cargo area which eats up too much valuable space, though the rear's low lift height is appreciated.
The ML55 impresses with its well-honed drivetrain. The TouchShift manual gear-change feature of the excellent five-speed automatic is a delight to use. A full slate of electronic aids are included, such as Electronic Stability Program, traction control, and Brake Assist System. Top speed is electronically limited to 146 mph, a moot point here in the States. Pushing the low-range button on the dash helps the ML55 clamber out of situations that would leave some SUVs awaiting a tow strap.
Both the X5 and ML55 have a button allowing the driver to "defeat" the stability program, but pushing it simply raises the level when it kicks back in. The BMW has a higher threshold before the computer intervenes than the AMG, allowing the X5 to be placed in a more radical attitude before electronics rein in the fun.
Verdict? These high-profile products from a pair of premier German automakers will find their way into different garages. Just don't buy one expecting the other. The X5 buyer will relish the visceral responsiveness, a sport sedan in all but physical dimensions. The ML55 was a state-of-the-art performance SUV when it debuted, and it still offers off-roading the BMW doesn't even pretend to. Yet in the real world, noting how these types of automobiles will likely be used, the X5 4.6is would be the one we'd be writing the (large) check for as best in this admittedly unique class.
2015 Mercedes-Benz M-Class SpecificationsVIEW ALL
|Fair Market Price||$44,750|
|Editors' Overall Rating|
|Mileage||18 City / 24 Highway|
|Horse Power||302 hp @ 6,500 rpm|
|Torque||273 ft lb of torque @ 3,500 rpm|