Jaguar XJ Supersport Speed Pack

What is it?

Just in case an artificial 155mph limit was insufficient to float your boat, Jaguar has introduced a Speed Pack for its range-topping XJ Supersport which cranks up the still-restricted top end to 174mph.

With a superabundant 510bhp already available from the familiar 5.0-litre supercharged V8 engine, no modifications are required to hit the new limiter, although the tyre pressure sensors and virtual instrument display have been reprogrammed to handle the bigger number.

The body, however, does get some minor tweaks to help with the aerodynamic demands of going 19mph faster. At the front there’s a splitter beneath the bumper that directs air beneath the car to reduce lift, and to the back the bootlid has gained a more pronounced lip spoiler.

Cosmetically, the Speed Pack car is differentiated by a black mesh grille, red brake callipers and a new 20-inch alloy wheel design. These additions are also bundled into a Sport Pack which is now avaliable across the XJ range, and includes a greater range of interior options.

What’s it like?

A guilty pleasure. Adding the Speed Pack to the XJ Supersport LWB clocks up a £97,495 sticker. That’s a serious outlay for a car whose distinguishing party trick can only be legally flaunted on derestricted stretches of the autobahn.

Tested on that surface, the Jaguar feels predictably at home. Immersed in its serene hum and flexing placidly over cracks and creases, it’s laughably easy to morph dull motorway miles into a mesmerizing 125mph blur.

The only real reason to punctuate its elegant stride is to attempt a child-like exploration of the remaining throttle pedal travel. The earnest, tail-wagging way the XJ reels in its new limit is something to behold - the V8 barely lets up its explosive consumption of fuel and air before it slides gently into its cushioned electronic barrier.

That appears at an indicated 285km/h, but the new limit is no less artificial than the old one. Not that your sanity would permit you to go much faster - the XJ feels appropriately at home beyond 155mph, but passing slower traffic like a fighter plane buzzing a container ship will be disconcerting to most drivers.

Should I buy one?

Despite its voracious thirst, it’s hard not fall head over heels for the Supersport. It’s big and brutish, but steers, cossets and entertains like nothing else in its class. Is the additional Speed Pack really worth the extra wedge? Rationally, no, of course not. In the UK fitting it would be as redundant as buying a beach towel in April.

But then the top-spec XJ itself barely registers on the commonsense scale anyway. It is too expensive and swaggeringly thirsty for all but the most prodigious budgets. Fortunately, those are exactly the kind of buyers likely to be persuaded by this boastful, four-figure line of ECU software code.

Jaguar XJ Supersport LWB Speed Pack

Price: £94,050 (£3,445 for Speed Pack); Top speed: 174mph; 0-60mph: 4.7 secs; Economy: 23.4mpg; CO2: 289g/km; Kerb weight: 1892kg; Engine: Eight cylinder, 5000cc supercharged petrol; Power: 510bhp at 6000-6500rpm; Torque: 461lb ft at 2500-5500rpm; Gearbox: Six-speed automatic