Stop us if you’ve heard this one before: There’s a rear-wheel-drive luxury sedan that’s also available with all-wheel drive, and its sticker price and curb weight are slightly higher while its performance and fuel economy are lower. The subject here is the Infiniti M56x, and true to the formula it’s a lot like the rear-wheel-drive M56. Consider that a good thing.

Opt to add a lower-case “x” to the badge of your M56, and you’ll leave the dealership $2500 poorer, as the base price is $60,915. Fuel economy drops from 25 mpg highway to 23, while the 16-mpg city figure is unchanged. (The difference in observed fuel economy was more pronounced, however, as we averaged 18 mpg with this M56x, five fewer than the 23 we saw from a rear-drive M56S.) Curb weight is up about 180 pounds, and the monster 420-hp, 5.6-liter V-8 under the hood pushes the all-wheel-drive car from 0 to 60 mph in 4.9 seconds, 0.2 second slower than the lighter M56S.

With a quarter-mile figure of 13.5 seconds at 108 mph, the M56x is only one-tenth slower than the M56S we tested for our June issue of the magazine. The two cars are dead even with an 11.4-second 100-mph sprint, and the M56x arrives at 130 slightly quicker. How does the heavier car achieve higher performance? Chalk it up to that first M56S being an early preproduction model; our long-term M56S actually beats the M56x at every 10-mph interval. But the takeaway is that the M56x doesn’t give up much in terms of pure acceleration.

The M56x’s braking performance, 166 feet from 70 to 0 mph, and skidpad grip, 0.83g, are close to those of the M56S. But the M56x shows better than its RWD counterpart in some crucial non-test situations. We have yet to drive an M56 without the Sport package, which comes with high-performance summer tires on 20-inch rims and a rear-wheel steering system. The M56x rides on all-season 18-inch tires and comes with a softer suspension. In strict handling terms, the M56S feels sportier, even if its numbers aren’t appreciably better. Meanwhile the M56x has a more natural steering feel, unlike the darty responses we’ve experienced in the M56S. The M56x also is far more livable on real-world roads. On the oft-criticized (deservedly so) potholed streets around our Ann Arbor office, the difference between the two cars is like the difference between sleeping on the floor and snoozing on a pillow-top mattress.

As we mentioned, the rest of the M56x is exactly like the M56S. Our test car came loaded with both the $3800 Deluxe Touring package and the $3000 Technology package, bringing the as-tested price to $67,715. For all that you get a 16-speaker Bose system and more electronic gizmos than you can shake a stick at, like lane-departure warning, forward collision warning, and adaptive cruise control. To borrow a phrase from the Marines, the Infiniti M56x makes more beeps by ten in the morning than most cars make all day. Fortunately you can turn all these systems off, although we’d advise skipping the Tech pack altogether. Inside, the M56x covers all the luxury bases with high-quality finishes, leather seats, and plenty of space in both front and rear seats.

Going back to the all-wheel-drive versus rear-wheel-drive script, the M56x doesn’t sacrifice much to take on the added weight required to send power to all four wheels, and it rides better than the M56S. But we’re willing to bet that the standard M56 without the Sport package delivers the same ride as this car. We’ve long been advocates of snow or winter tires in lieu of all-wheel-drive systems, and this case is no exception. But if you must have an M56 with the “x” badge, rest assured that you’re not sacrificing much to gain what amounts to peace of mind.

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