We already know that the Jag XF is a star, but it has to be up with the BMW 5-series to be taken seriously. Andrew Frankel decides if it is.

Partly because it was the professional thing to do, but mainly because I was bored rigid at 40,000ft on my way to Arizona for my rendezvous with the Jaguar XF, I whipped out its spec sheet and compared it with that of the BMW 550i against which we planned to test it the following day. I'll say now that it didn't make pretty reading.

And if you'll bear with me, I'll put you in the same picture I found myself in before climbing aboard; it helps give a sense of scale to the mountain that Jaguar's most important new car of the last generation now must climb.

In the past, one of the few advantages you could always rely upon a Jaguar having over its competitors is price; ever since the XK120 appeared in 1948, Jaguar has always undercut the opposition to such an extent that it has become part of its brand.

Not any more, it seems. The XF waiting for us in Phoenix to face the 5-series is the slightly tortuously entitled V8 Premium Luxury, which carries a price tag of £45,500, compared with £45,745 for the 550i SE. And no sale at this end of the market ever fell through over a paltry £245.

BMW, in its wisdom, has chosen to provide us with a 550i M Sport which changes its price (to £48,305), but not the underlying sense that Jaguar has jacked the price of the XF through the sky. Fact is, this normally aspirated XF not only costs more than the S-type it replaces, but more even than the supercharged S-type R that used to top the range.

True, both XF and 550i use normally aspirated V8 engines, but while the Jag's displaces 4.2 litres, the BMW's has 4.8 litres so, unsurprisingly, it has rather more power and torque, making it much quicker. Jaguar claims a 6.5sec sprint to 62mph for the XF while BMW claims 5.3sec, even when fitted with an automatic gearbox like the one that comes as standard in the Jag. Yet astonishingly, it is the BMW that uses less fuel on all quoted cycles and emits less CO2 as well. It is also a fraction lighter.

Which brings us to a fairly blunt bottom line: there is not a single headline figure the most tedious type of pub bore would dream of quoting in the local juicer that puts the Jaguar on top; if you set a lot of store by statistical evidence, the BMW has won this test without turning a wheel.

Even if you don't, the task facing the Jaguar today remains demonstrably massive. This, after all, is the 5-series, a breed of BMW that has remained on top of its class through its last three generations dating back to 1988. Would you, knowing the desperate financial straits Jaguar has been in throughout the development of the XF, bet on it being the one to topple it?

But don't conclude that this race is already run – not yet, at least. You have only to look at the next most recent Jaguar, the XK coupé, to know that Jaguar retains a perhaps unrivalled ability to make up in design flair and engineering ingenuity what it has so painfully often lacked in folding readies. On paper, an XK coupé seems just as disadvantaged against a 650i coupé as does the XF relative to the 550i, yet there's not a person with a pulse in this office who'd walk past the XK to get to the 6-series.

Parked beside it, the Jaguar XF is startlingly more svelte than the 550i. Although it is externally bigger than its rival in every important dimension, the Jag appear to be more lithe and sleek, thanks to its clever four-door coupé design and despite the BMW's M Sport bodywork.

Which is not to say the XF is perfect; I like almost everything about it apart from the nose in general and the headlights in particular, which simply seem not to fit the rest of the car. You notice it most in strong or dark colours. This car is not like the XK, a car that can appear a little odd on the printed page but which always looks gorgeous in the real world. If the nose of the XF seems awkward to you, that's because it is. And it is Jaguar's simple good fortune that the 5-series, for all its many and manifest talents, is so externally inelegant.

Inside, however, the XF makes no mistakes with its appearance. Jaguar design director Ian Callum says as much time was spent working on the interior of the XF as its exterior and frankly it shows, for there is no classier cabin in the segment.

The dashboard is a particular triumph, comprising classy dials, a swooping fascia and the usually uneasy alliance of wood and aluminium working together with rare harmony. By contrast, the 5-series' driving environment looks functional but dull, lacking entirely the sense of occasion offered by the Jag's interior. It's a place to help you go about your business, but never does it make you feel at home like the cabin of the Jaguar.

Unless, of course, it is your unlucky fate to be stuffed into the back seat of the XF. Both Jaguar and BMW have larger saloons to sell, so it's no surprise that neither offers space to sprawl, but while the 5-series offers adequate accommodation for four decent-sized adults, the XF does not, lacking both head and legroom.

Jaguar is keen to point out that there's a lot more room in the back of the XF than there was in the S-type, but that's like saying measles are better than mumps: true, but you'd rather not have to put up with either. Jaguar also claims the XF has the biggest boot in the class, but fails to mention this is only true if you give up the right to a spare tyre. With one in place, the BMW's trunk is larger, and it has run-flat tyres as standard.

Then again, perhaps we are looking at the XF the wrong way. Its chief engineer, Mick Mohan, is fairly frank on the subject, saying, "We are not in the transportation business. We are in the entertainment business." That sounds like one of the cheesy excuses for building a cramped car that we hear from time to time. But then you actually get this XF out onto the open road.

I'll deal with the difficult bit first. The BMW has such a power advantage (362bhp vs 294bhp) that even if its chassis were useless it would probably be able to drive away from the XF on a decent road. But as we all know, the 5-series chassis is extremely accomplished. Working with typical fluency with that engine, it confers a level of point-to-point speed that, while not enough to embarrass the Jaguar, is nevertheless substantially superior.

Whether that is a significant advantage or not depends on how much you value driving flat out to arrive at any given place a few seconds sooner than you would had you bought a Jaguar instead. I've always been more interested in the journey than the arrival time, and regardless of whether we are running fast down the Interstate, flinging the cars up a mountain road or snarled in city traffic, the Jaguar's driving seat is always the place I want to be.

Away from the hard numbers and into the less easily defined but no less important world of how a car feels and how it makes its driver feel, this particular XF offers an enticing experience beyond anything the BMW can match.

The steering, for instance, is as good as any I've tried in this class of car. Instead of appearing to react to your action, so that your progress is defined by the process of input followed by deflection, it flows as seamlessly as if your arms were steering the front wheels themselves. Good though the Beemer's steering is, it can't do that.

The XF also rides with rare equanimity, and is untroubled in this area by the BMW with its needless M Sport suspension. If the firmness of the 550i's set-up over that of the XF was matched by a commensurate advantage in poise over a difficult road, then there would be some sense to it; there's not.

Although the XF is substantially more supple, it controls its body movements just as well, and if its grip levels are not quite so high, the feel of its chassis is such that it can be driven with even greater confidence than the still-excellent 5-series. And right on the limit, while the Jaguar has a little more inherent understeer than the commendably neutral BMW, it executes throttle-induced changes of direction more neatly, and feels at least as nimble as a result.

But it is the powertrain that springs the biggest surprise. BMW furnished us with a manual 550i, which did its case no favours at all because the change is notchy and unsuited to the torque characteristics of the engine. But we suspect that even with the same ZF auto offered as standard by the Jaguar, the BMW would still feel less special to drive, despite its superior performance.

Jaguar can make this gearbox do things that none of the other brands that use it can dream of. Its changes are snappy and smooth in both directions, and when you use the paddles on the steering column to call up an instant downshift, the speed with which it reacts, while perfectly blipping the throttle to match the revs, is reminiscent of a DSG twin-clutch gearbox; it really is that good. Yet set the controls to cruise and you at once forget the car has a transmission at all, unless you happen to spot the occasional repositioning of the rev-counter needle.

The speed with which the 'box reacts also means you are rarely (if ever) aware of the V8's comparative lack of low-down torque. The engine is so sweet and sonorous in its upper reaches, and the gearbox so eager to keep it there when you're pressing on, that holding the motor in the 4500-6000rpm band (where it works best) isn't just easy, it's natural. The BMW engine will deliver greater grunt at all points in the rev range, but it's never as easy on the ear or quite so free-spinning at the top end.

So it is because the XF is such a delight to sit in (up front) and drive that I'm going to give it the nod here, and the designers and engineers at Jaguar who have sweated so hard for so long, with so few resources at their disposal, are right to feel proud.

Building a car to beat a 5-series is a task that has foxed many of the industry's greatest brains for most of the past 20 years, but Jaguar has pulled off what to many must have seemed near enough impossible. It should also be said that the normally aspirated V8 XF is a clearly superior product to its more expensive, less likeable supercharged sister.

But that is not the only issue that qualifies this victory, for in truth it asks as many questions as it answers. What will the XF be like, not in the Arizona desert, but on British roads? How will it stack up against the Mercedes CLS, a car whose four-door coupé design was clearly studied hard by Jaguar? Above all, how will a diesel XF, the version that will outsell all others put together in Europe, compare against its strongest German rivals?

We look forward to providing answers to all of these questions and many more in the next few weeks and months. In the meantime, I would not be so mean as to begrudge Jaguar its victory here; it is deserved for sure. But the real test is yet to come.

BMW: DOWN BUT NOT OUT

So finally, a Jaguar that can put a BMW in its place. But there's a new 5-series coming in 2009 — it's already undergoing prototype testing in Germany — and while the current 5-series outguns the XF on paper, the next one will further widen the already sizeable specification gap between the two cars. It will have the new 401bhp twin-turbo V8 launched in the X6, with an eight-speed gearbox, and a line-up of exceptionally clean and powerful diesels.

Whether the 5-series will be able to match the Jaguar's considerable emotional appeal is impossible to say, but BMW won't let its position as the producer of pre-eminent driver's cars be usurped for long. The new Five will take the fight back to Jaguar.

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