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Despite the new name, the GLS isn’t, strictly speaking, a new car.

Instead, it’s a facelift of the Mercedes GL-Class that’s been around since 2012. The oily bits have largely been left alone, apart from the adoption of a new nine-speed automatic gearbox for the 3.0-litre V6 diesel, which makes up the vast majority of sales.

Although the engine still produces the same 255bhp, the extra ratios mean economy and emissions are improved. It’s also fairly brisk considering the 2.5-tonne heft of the GLS, with 0-62mph taking a respectable 7.8sec – a small improvement on the outgoing GL-Class.

If you want to go faster, you can still get an AMG variant with a twin-turbo 5.5-litre V8 producing 577bhp and peak twist of 560lb ft propelling the big SUV to 62mph in 4.6 sec and on to an electronically limited 155mph. If you opt for the AMG Driver’s pack this top speed can be increased to 168mph.

The rest of the changes are cosmetic or simply make the GLS even more lavishly equipped than the GL-Class. Up front is a new nose that mirrors the rest of the Mercedes-Benz range, while the rear gets redesigned exhaust tailpipes. Inside there is a new steering wheel and instrument panel and a redesigned centre console.

Three trims are available – AMG Line, designo Line and the AMG breathed on GLS 63. The entry-level model gets 21in alloy wheels, adaptive air suspension and LED headlights, a panoramic sunroof, a sports braking system and an aggressive AMG bodykit on the outside, while inside there is a 360-degree camera, tri-zone climate control, heated seats all round, a Harman and Kardon sound system, and Mercedes-Benz’s 8.0in infotainment system with DAB radio and Bluetooth connectivity.

Upgrade to the more luxury appointed designo Line and your GLS gains Mercedes’ active curve system, limiting body roll and increasing agility and safety. There is also ventilated and massaging front seats, four-zone climate control, temperature-controlled cupholders, rear sunblinds and a luxury quilted leather upholstery.

Looking for more speed in your life then the GLS 63 may very well be the answer with its aggressive bodykit, high-performance braking and exhaust system and AMG-embossed sports seats to go alongside the 5.5-litre V8 at the front. In this test we sampled the mid-range model with the default diesel motor on UK roads.

If there’s one word that sums up the GLS, it’s 'massive'. Compared with an Audi Q7, the Mercedes is longer, taller and a bit wider. Although that might sound like a recipe for disaster on our tiny island, the GLS comes as standard with a 360deg camera system that makes manoeuvring surprisingly easy. The boxy body also helps.

On the move the V6 is as smooth and refined as you’d hope from a luxury SUV, no doubt helped by its sub-2000rpm cruise. Sure, when pushed you do hear it, but it’s not an unpleasant noise which avoids the more strained sound of a Q7’s V6 diesel.

Over larger bumps and undulations, the GLS strikes a fine balance between comfort and body control at speed, largely avoiding the floaty feeling you get from some air-suspension systems. You do pay a price for standard-fit 21in wheels though, with expansion joints and potholes thumping through the car's structure.

Designo Line benefits from clever anti-roll bars that all but eliminate body roll in the bends. Although this masks the bulk of the GLS to a point, you do feel the car’s mass if you push harder, especially during braking.

The steering is precise enough but is missing any real feedback, even when put into Sport mode. The suspension also gets a Sport setting, which firms it up significantly. You feel much more of the road but it seems pretty pointless considering the absence of roll in Comfort. 

Step up into the cabin and you’re treated to a commanding view of the road over the long, vented bonnet. There’s ample room to stretch out for all occupants in the first two rows while even adults in the rearmost seats shouldn’t complain too much. Compared with rivals such as the Range Rover Sport, there’s noticeably more space for third-row occupants.

Although the boot is relatively small with the third row up, the seats fold electrically into the floor as standard. Do this and you’ll open up a vast cargo area, but fold the middle row down too and there isn't much you couldn’t fit into the rear of the GLS.

It’s not all good news though. Considering that Mercedes bills this as ‘the S-Class of SUVs’, some of the interior doesn’t feel as plush as you’d hope. While there are plenty of nice leather and metal trimmings, there’s a surprising amount of creaking plastic and buttons which have come from much lower down in Mercedes' range. Next to the considerably cheaper Audi Q7, it’s disappointing.

If you’re after a big, imposing SUV, there’s not much on sale that is either as big or, indeed, imposing as the GLS, save for the (more expensive) full-fat Range Rover of course. The problem is that while it’s decent to drive and has the looks on the outside, it feels underwhelming in places on the inside.

If you're after seven seats, we’d still take a look at the Range Rover Sport first, as it proves even nicer to drive for similar money. Alternatively, try the much cheaper Audi Q7; its interior is even better and it’s certainly not short on space.

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