The FX, as well as being the best-selling Infiniti, was also the first of the brand's products to be fitted with the three-litre diesel engine which is now being used elsewhere in the range. The petrol alternatives - a 3.7-litre V6 and the silly but very popular five-litre V8 - are much more powerful, with 315bhp and 385bhp respectively compared with the diesel's 235bhp, but in this form the FX gives you as much straightline performance as you need. Anything else is just noise and wasted fuel.On the subject of fuel, the FX30d uses much less of it than its petrol-fuelled relatives. The official combined consumption figure is 31.4mpg, and although there was no opportunity in this test to measure what the actual figure was, it pleased me note that the trip computer credited me with around 35mpg after several hundred miles of relaxed driving on motorways and A-roads.However, the FX has another function called Eco History which tells you how much fuel you've used at various stages on your journey. According to this I was rarely doing better than 30mpg. Something wrong there. The Eco History and the trip computer might both be wrong, but they can't both be right.As well as providing decent amounts of power from low revs, the engine is quiet enough most of the time, if a little rattly when cold. Both this and the very supportive front seats helped to make it a pleasant cruiser, though there are other elements which work against that. The ride, for example, can be either slightly wallowy (because of underdamped suspension) or slightly jittery (because of low-profile tyres on huge 21" wheels) or, in the gloomiest of scenarios, both at the same time. On smooth roads it's fine, but I don't need to remind you that we do not live in a country overly blessed with those.The seven-speed automatic gearbox of the test car could be a little jerky on downshifts, though that might simply mean that it needed adjustment. Also odd was the way the electronics wouldn't let the engine revs fall below about 1500rpm before ordering the transmission to select a lower gear, and preventing manual selection of the higher gear. This seemed strange, since the engine felt as if it would run quite happily at lower revs, using less fuel in the process.As the heading to this article suggests, the test car was in GT Premium specification, which basically means that Infiniti throws absolutely everything at it with the exception of alternative paint, which costs a further £679. A list price on the high side of £51,000 may seem slightly alarming to those of us not accustomed to paying so much for a car, but at least you can be confident that the manufacturer won't sneak in several thousand pounds' worth of extras.My favourite piece of equipment is the Around View Monitor, in which four cameras present a combined image on the central display screen which makes it look as if there is just one camera mounted about ten feet above the car. Accurately placing the FX in tight spots - impossible if you try to use the rear windows, because they are stupidly small - becomes relatively easy, though there must surely come a time when something in the system fails and requires expensive repair.I also like the satellite navigation, which allows you to input address details using a virtual QWERTY keyboard (why don't more manufacturers do this?) and has very good, clear mapping.In fact, I think Infiniti's strength is its ability to provide these ancillary features. The basic car is not my favourite among large SUVs - of the diesel models considered by Infiniti to be the FX's direct rivals, the Porsche Cayenne is slightly better to drive, while the Range Rover Sport is in a different league entirely. And, for those of you who think this is important, both offer very much more luggage space.A further point in the FX's favour is its rarity. At the time of writing there are still only six dealerships (beg pardon, "Infiniti Centres") in the UK, and the FX makes the Cayenne and the Sport look almost like mainstream models. All the same, if I were going for a car like this I'd want to buy the one that suited me best, not simply one that was noticeably different from everything else. Engine 2993cc, 6 cylinders Power 235bhp Transmission 7-speed automatic Fuel/CO2 31.4mpg / 238g/km Acceleration 0-62mph: 8.3 seconds Top speed 132mph Price £51,290 Details correct at publication date