It's been a couple of years now since Mercedes-Benz launched the third generation of its luxury SUV ML-Class and, if you read our first test (a ML250 BlueTEC in September 2012), you would’ve noted we weren’t all that enthused.

We detected a whiff of B-Class about the new W166’s exterior design and thought the 2 143 cm3 turbodiesel somewhat underpowered for a car of this considerable bulk. All in all, we felt that the car wasn’t much of an improvement on its more aggressively styled predecessor.

However, having driven more of its derivatives, it is certainly a model range that’s grown on us. And the ML400 with its new petrol V6 bi-turbo has only reinforced that change of mind. We first experienced this new V6 in the facelifted E-Class where it drew high praise and the new engine is claimed to be both more efficient (meets EU6 emissions standards) and, thanks to the bi-turbo, more
powerful than the ML350 Blue-Efficiency. Whereas the older naturally aspirated 3 498 cm3 unit generated 225 kW and 370 N.m of torque, the new engine puts out 245 kW and 480 N.m across a wide
range of 1 600 to 4 000 r/min.

Behind the wheel, this increase in power isn’t immediately obvious. Rather than anything that might snap your head back, the increase in power is delivered smoothly and progressively, and the SUV always feels like it’s got plenty in reserve for any mid-gear overtaking manoeuvres on the highway.

Interestingly, our fuel run didn’t quite back up Merc’s efficiency claims over the previous model and the ML400 returned a figure of 11,4 L/ 100 km. We didn’t test the previous ML350 BlueEfficiency, but on our fuel index it boasts 10,2 L/100 km, whereas the ML400 is 11,5 L/100 km.

For a big SUV, this is certainly within acceptable limits and up against its other qualities, this quickly fades as a factor. This engine is so quiet you’d be fooled into thinking you were driving a hybrid. The buttery smooth power delivery is complemented by a seven-speed 7G-tronic auto ‘box.

Whatever noise the drivetrain might make is blocked out by excellent NVH control, while the levels of interior build quality are up there with Benz’s luxury sedans. That said, the spec is fairly spartan – air suspension, reverse camera and a sunroof are pricey options – and the interior design leans towards the solemn.

Compared with its rivals, interior space is excellent. Where it’s also measurably better than its rivals is out back, with 384 dm3 of luggage space; that’s bigger than the BMW X5 (368 dm3) and the same as the Porsche Cayenne. Lower the rear seats and it takes the honours with a whopping 1 512 dm3.

If you’re looking for an SUV that can occasionally satisfy that sportscar demon sitting on your left shoulder, then look at the equivalent X5, Cayenne or Range Rover Sport. If, however, you’re looking to ferry your family in luxury and refined comfort, then the ML400 makes a compelling case.