Every time I see someone driving along a motorway in a large SUV a voice inside my head starts asking peevish questions about why anyone would choose to do such a thing. Having spent a couple of days doing exactly that over nearly a thousand miles in a Range Rover Sport SDV6 I can see the appeal.The current Sport (unlike the one on sale until the middle of 2013, which was based on the Discovery) is closely related to the Range Rover proper, and it's every bit as talented in difficult off-road situations. I didn't explore that during this test, but I've done so previously on routes so treacherous that at one point I got out at the top of a fearsome slope and let someone else drive down it while I slithered through the bushes.It wasn't that I didn't think the Sport would get to the bottom safely. It was just that I didn't want to be sitting inside it when it did. Naturally, the car made a better and less messy job of the descent than I did, and the driver was both completely unruffled and slightly amused when I climbed back in wearing what now appeared to be some sort of elaborate camouflage.That's how good the Sport is off-road. But parent company Land Rover stopped basing its marketing of the full-sized Range Rover on this kind of ability several years ago, and it's not devoting much effort to that with the Sport either. Both are being promoted as luxury cars.Quite rightly too. The sense of refinement generated by these lumbering heffalumps is extraordinary - not so much because of the digital instrument panel, which is far from being the best on the market, and certainly not because of the manual paddles for the eight-speed automatic gearbox, which are ridiculously cheap and tacky for a car like this, but absolutely because of things like the incredible lack of wind noise.There should be much more of this than there is. In aerodynamic terms the Sport is a thundering great lump, and the airflow over the enormous door mirrors alone should be one of the dominant noise sources at anything much over town speeds. But, thanks to excellent acoustic control, it simply isn't.By rights, there should be a lot of engine noise too, but, again, no. It used to be the case that diesel Range Rovers were very much louder than petrol ones, which themselves made quite a racket. The 288bhp SDV6 diesel in the Sport is as surprisingly subdued as the 503bhp supercharged five-litre petrol V8 available at very much greater expense, and in normal road use they actually sound very similar.Up to a point they also make the Sport behave in much the same way. It's only when you want to accelerate very hard indeed that the diesel loses out, and while it's nice to have the V8's extra performance, and no doubt to be able to demonstrate that you can afford it, I'd struggle to come up with any everyday motoring situation in which you would need it. The diesel is quick enough as it is.Fuel economy is at a level that would have been considered impossible at the turn of the century. The official combined figure is 37.7mpg, and the trip computer told me that I'd nearly matched that with 37.3mpg. That reading seems to have been very optimistic, since I measured 32mpg, but that, if true, is still pretty good for something with so much power and aerodynamic resistance.It's also probably not much of a consideration for anyone who can afford the car in the first place. If that means you, you may well be attracted by the badge as much as anything else, but to concentrate on that would be to ignore the fact that this is very comfortable and extraordinarily high-quality vehicle with - if you accept that there's a need for cars like this at all - hardly anything to criticise. Engine 2993cc, 6 cylinders Power 288bhp Transmission 8-speed automatic Fuel/CO2 37.7mpg / 199g/km Acceleration 0-62mph: 7.1 seconds Top speed 130mph Price £61,250 Details correct at publication date