It’s fitting that the GLS is assembled in the spiritual homeland of big-boned SUVs; specifically, at Mercedes’ Tuscaloosa plant in Vance, Alabama. And this widely redeveloped model does cast quite the shadow, being 77mm longer and 22mm wider than the previous GLS and with a footprint comfortably larger than that of its closest rivals, the BMW X7 and Range Rover.

Indeed, it dwarfs conventionally big cars such as the Volvo XC90 and has a wheelbase 100mm longer than that of even the long-wheelbase S-Class. Is that too big – at least, for UK roads? We’ll see, but to go bigger in Europe, you would need the Rolls-Royce Cullinan.

It will be a modest step to get into the GLS for most people, and a big one for some – particularly if the air suspension is at its highest setting – so these running board steps come in very handy. They look the part on the car, too.

As is commonplace for models that could more accurately be described as high-riding limousines than off-road vehicles, the GLS’s construction is more S-Class than G-Wagen. Each axle uses pneumatic springs and, although those springs are controlled with double-wishbone suspension at the front axle, the rear uses a multi-link arrangement. It’s all attached to Mercedes’ Modular High Architecture, which is largely the same as the Modular Rear Architecture used by the S-Class, E-Class and C-Class saloons.

This hardware lays the ground for cutting-edge autonomous driving features and is said to weigh less – at 2415kg, the claimed weight for this GLS 400d 4Matic makes it 40kg lighter than the car it replaces. The 400d will remain the only powertrain available to UK buyers until the Mercedes-AMG 63 (twin-turbocharged V8) arrives later this year.

The 400d uses the same 2925cc ‘stepped-bowl’ straight-six twin-turbo diesel available elsewhere in the Mercedes range – and highly rated it is, too, for its response and refinement. Mated to Mercedes’ nine-speed gearbox, it delivers 326bhp and a robust 516lb ft and does so with new-found versatility. Whereas the old GLS powertrains used a fixed 50:50 torque split, the new powertrain is variable, defaulting to rear-wheel drive for sweeter drivability on the road but able to shift half the torque forward via its centre differential.

The two most notable option packs are the Off-road Engineering Package, which is fitted to our test car, and E-Active Body Control, which uses stereoscopic cameras (that is, two feeds rather than one) to scan the road ahead and prime the dampers – and that, for some reason, isn’t available in the UK. The Off-road Engineering Package adds a low-range transmission, off-road ABS and the ability to lock the driveline’s central coupling.

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