The Range Rover celebrated its fortieth birthday last month. That says a lot about the soundness of the original concept of a luxury 4x4 - but even more about the way Land Rover has developed it.

There have been just two completely all-new successors to the original car, which stayed in production for the best part of a quarter century; the only way Land Rover has been able to keep up with, or for the most part, stay ahead of, German and Japanese rivals with much shorter model replacement cycles, is by constantly upgrading the cars during their entire production lives. The last version of the original "classic" Range Rover, with its four-door body shell, long wheelbase, air suspension, automatic gearbox and lavishly-trimmed interior, is quite different to the rather bare 1970 original, even if they are still recognisably the same car.

The current Range Rover was introduced in 2002, which probably makes it one of the older current models on sale, but it is very difficult to identify any area in which it is really showing its age. That's partly because the 2002 car was such a big jump forward compared with its predecessor – and most of the competition – but also because of that process of constant upgrading, which for the 2011 model year has mainly been focused on the diesel version.

Diesels have been available on Range Rovers since 1986, but it was only three years ago that Land Rover's top car finally got a diesel engine that delivered the same sort of power and refinement as the big V8s that have been fitted to petrol-driven versions since the beginning. That engine, too, was a V8, whereas previous diesels had been smaller fours and sixes, often bought in from other suppliers. It had a capacity of 3.6 litres, and was a superb match for the Range Rover; it provided the car with a lot more performance, of course, but it was the smooth, refined manner in which it delivered that performance that really impressed.

But the pace of development in diesel technology has accelerated during the last decade, and by last year, a big revamp to the smaller V6 diesel fitted to Land Rover's other big cars had cut away most of the V8's performance edge; this year, however, the V8 has undergone its own big programme of improvements, similar to that carried out on the V6, and also benefits from an increase in capacity to 4.4 litres. This improved engine is now paired with an eight-speed ZF automatic gearbox and the results are predictably impressive; the 2011 diesel Range Rover operates almost silently most of the time, but when the engine needs to work hard – which is rarely, given the amount of power on hand - it sounds and revs more like a good petrol V8, rather than a diesel.

There are a number of other changes besides the new diesel powertain – a rotary gear selector similar to that found on cars built by Land Rover's sister company Jaguar, for example – but in most other areas, the 2011 Range Rover is the same extremely appealing package as before, delivering very high standards of comfort and on-road behaviour, as well as the outstanding off-road abilities that really give Land Rover's cars their edge over competitors; it is, perhaps, a pity that most customers won't experience this side of the Range Rover's performance directly, even if they are reassured by its presence. This thing really will climb apparently impossible gradients or wade deep streams with rock strewn beds, and simply soaks up the sort of general cross-country punishment that would surely break some other manufacturers' cars.

So even after forty years, it appears that the Range Rover story still has plenty of life left in it. Once just the icing on the Land Rover cake, Range Rover has for years now been the main focus for the company's development, becoming in the process a premium automotive brand, rather than just the name of a single model. The first big move to make an entire Range of these Rovers came in 2005, with the introduction of the less expensive Discovery-based Range Rover Sport. The Sport has been highly successful in attracting new classes of buyers but still belongs in the same size and weight category as the original car.

Perhaps of greater significance was the unveiling last week of the Evoque, a baby Range Rover designed to appeal to an even wider audience. Roughly the same size as the existing Land Rover Freelander, the Evoque carries into production next year, almost unchanged, the dramatic styling of the LRX concept car. The Evoque will be available with a two-wheel drive transmission as an alternative to a full all-wheel drive system, a move that would probably have been unthinkable only a year ago, before other manufacturers such as BMW with the X1 and Kia with its latest Sorento started offering a 2WD option on their SUVs. That raises the question, of course, of whether with the continuing expansion of the luxury Range Rover brand, Land Rover might eventually move too far from those rugged off-road roots that have been so important in giving it an edge over other makers of SUVs. But if that is a problem at all, it is a problem of success.

2011 Model Year Range Rover 4.4 litre TDV8

Price: £81,500

Engine/transmission: 4.4 litre twin-turbocharged V8 diesel paired with ZF eight-speed automatic transmission and Land Rover permanent four-wheel drive system

Top speed: 130mph

0-60mph: 7.5 seconds

Consumption (official combined cycle test): 30.1mpg

CO2 emissions: 253g/km

Also worth considering: Audi Q7, Mercedes GL-Class, Range Rover Sport

Also worth considering: Audi Q7, Mercedes GL-Class, Range Rover Sport

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