Life is far from great at Jaguar right now. Its X-type bulk seller is performing below expectations and it has no diesels to offer when diesel has become the latest fashion. Meanwhile Audi, BMW and Mercedes continue to make packets out of peddling cheap-to-develop “lifestyle” estates, of which Jaguar has none. Nor has the mood in Coventry been helped by three hideously expensive and fairly disastrous years in Formula One.

And on the surface this new XJ looks far from the solution to any of these problems. Cautiously styled to resemble an expanded version of the old XJ, it appears too keen not to offend those who have made sure Jaguar has sold more XJs over 35 years than every other model put together. It should have been as progressive and beautiful as the original XJ in 1968, but it appears retro and introspective.

In fact, contrary to appearances, the XJ is the most radical big saloon on the market and Jaguar has timed its appearance to perfection, just as its competitors are starting to focus on the very issue it addresses.

The issue is weight. By building the XJ around an aluminium monocoque and clothing it in aluminium body panels Jaguar has created a phenomenally light car.

The heaviest XJ, the supercharged XJR, weighs just 1,665kg compared with, say, the 2,319kg of the heaviest VW Phaeton. That’s two-thirds of a ton less weight to carry around. And even against more slimline rivals you’d need to bolt a baby elephant to an XJ’s roof to balance the scales.

So it will come as no surprise that, as a range, the new XJ offers not only the best average performance of the class but also the most frugal fuel consumption and the lowest emissions. It has also addressed the one critical failing of the old XJ: the new car is as spacious as its predecessor was cramped.

Jaguar’s enlightened philosophy has also made it possible to start the range with a cut-price, £39,000, 3 litre V6 that will still hit 60mph in 7.8sec on its way to 145mph.

Next up is a new 3.5 litre V8 motor that provides the XJ8 only a little more power and performance (0-60 in 7.3sec, 150mph) for what seems like a vast hike in price to £48,000. What these statistics don’t show is not only the much better level of standard equipment offered by this car but also the superior manners of its eight-cylinder engine.

Even so, those who can spend just £3,500 more on the 4.2-litre version of the XJ8 will probably not regret it. This time the performance hike is considerable (0-60 in 6.3sec, top speed restricted to 155mph).

But the most powerful of the new XJs is the XJR, whose supercharged 400bhp engine provides genuine supercar performance: 0-60mph in 5sec dead and a top speed that would top 170mph were it not for electronic intervention. This is Porsche 911 performance in a four-door saloon. It costs £58,500.

But while the XJR is easily the quickest of the new range, it is far from being the best. The Mercedes E55 AMG — with considerably more power and torque — would blow it off the road. And while it is more capable than the old XJR, its handling has been tamed and some of its unruly charms lost in the process.

At the other end of the range, the new XJ6 makes a more persuasive argument. At 1,545kg it is lighter than a fully equipped X-type, a car designed to compete with the BMW 3-series. As a result, the XJ6 is so light on its feet it disguises its Mercedes S-class size brilliantly. And don’t worry that its engine is derived from a Ford Mondeo unit: it’s smoother and quieter than Mercedes’s similarly powerful V6 engine.

But the real superstar is the XJ8 4.2. More than quick enough for most tastes yet unsaddled with the expectation to perform, it is as smooth, sophisticated and capable as you could ever hope a Jaguar to be.

Best in class then? Not quite. The Mercedes S-class still holds that title with both hands. Ultimately, these are luxury cars and what matters most are ride, refinement, luxury feel and appearance, and in all these the Mercedes still beats the better handling, more efficient Jaguar. But it’s close and Jaguar should be congratulated. When you’re up against the S-class, there is no shame in being as good as the best of the rest.

VITAL STATISTICS
Model Jaguar XJ8 4.2
Engine type V8, 4196cc
Power 300bhp @ 6100rpm
Torque 0 lb ft @ 3100rpm
Transmission Six-speed automatic
Suspension (front) double wishbones, air springs, anti-roll bar; (rear) multi-link axle, air springs, anti-roll bar
Tyres 235/50 ZR18
Co car tax £7,209 for a higher-rate taxpayer
Top speed 155mph
Acceleration 0 to 60mph: 6.3sec
Price £51,500
Verdict Disappointing looks disguise a fine new car