Forget the V8 F-Type or that gauche XFR-S. This is what a fast Jag should be. This is the XJR, which sees Jaguar's most powerful, wildest-eyed V8 stuffed into its biggest, cruisiest car. On paper, it's a recipe for weirdness. On the road, it's anything but.
See, the problem Gaydon traditionally faces as it ventures to the sporty end of the market is that the things we ask from fast cars - muscular handling, taste-the-tarmac feedback - are at odds with the stuff Jag does so well: feathery ride quality, syrup-smooth gearboxes, lightness of touch. Which leaves the fastest Jags feeling a mite disjointed: neither fish nor fowl but a strange turkey-salmon hybrid.
But the XJR allows Jag to have its artisan Battenberg and eat it: to meld seamless road manners with that maniacal supercharged V8. The visual upgrades are subtle: chrome-tipped air intakes and bonnet louvres up front, delicate bootlid spoiler and quad exhausts at the rear. Beyond that - and certainly compared to that look-at-me-I-can-go-fast XFR-S - the effect is pleasingly stealth.
Until, that is, you unleash Operation Heavy Right Shoe and discover the XJR is genuinely, shockingly fast. The sort of fast that, when you indulge in a dose of full-bore acceleration for the first time, has your passengers diving for grabhandles and burying their weeping faces in the XJ's lush leather. Jag quotes 4.4secs for the 0-62mph run, which is lightly mindblowing for a car of this size. But the XJR feels even more rapid than four-point-four. Maybe it's a touch of conservatism from the boys in the Impressive Numbers Department. More likely it's the delicious incongruity of a two-tonne, lavishly upholstered houseboat that'll launch at the middle distance like a stabbed wildebeest.
Another number. Jaguar says the XJR will accelerate from 50-75mph in under 2.5 seconds. Just wrap your brain around that one. Cruising at 50mph. Press throttle. One-thousand-two-thousand-seventy-five-mph. At the British speed limit in the time it takes to clear your throat. And the worrying thing is, I'd wager the Jag will get from 70 to 95mph or, say, 125 to 150mph just as quickly. I'm not sure I've ever driven anything that'll shove itself so fast with such caddish insouciance. "Me? Doing triple-figure speeds across Lord March's lawn with Bach blaring from my speakers? No comment..."
Getting too finicky about the finer handling characteristics of something so inherently silly seems to miss the point a little, but suffice it to say the XJR will get round any corner as fast as any normal human dares. With springs some 30 per cent stiffer than those of the standard XJ, and active damping that adjusts 100 times a second to keep any vertical movement in check, the XJR remains composed on even the nastiest stuff. The overall impression is of a car far lighter and smaller, one curiously capable of shrink-wrapping around you. No, it isn't as hands-on as a Porsche Panamera. But if you want hands-on, why are you looking for a V8 limo?
And if we're talking rivals, true, the new S-Class will dish out a kicking to the big Jag in the gear'n'gadgets department: the XJ's infotainment feels faintly Neolithic nowadays. But I can't believe even the S63 will boast the chassis balance and finesse of this Jag.
Because here's the XJR's neatest trick: it manages all that last-rites-inducing fastness without forgetting its raison d'être: to waft four large humans along in silent, rapid ease. Fold your inner Stig into the glovebox and engage Respectful Chauffeur mode, and you'll find the XJR will breeze with the best of 'em. Maybe, if we're being picky, the big wheels and harder suspension have robbed a few handfuls of down from the standard XJ's featherbed ride, but it still proceeds with a waft beyond the reach of any of those vulgar Germans. There's just enough of a yoghurty boom from the V8 to remind you - and the plebs you pass - that you're in something a little more... deluxe than yer run-of-the-mill diesel limousine, but mainly the XJR is filled with the deafening roar of expensively damped silence. It's the sort of car in which you reach the end of Now That's What I Call Bagpipes Volume VII to find, with a start, you're at the other end of France. It is faintly magical.
And look, even we, with our shaky grasp on car-buying reality, realise posh V8 limos are among the nichest of the niche persuasions. The standard diesel XJ is fast enough for even the most speed-hooked limo buyer, which means the XJR is pitching for the most weirdly specific market since the ill-fated launch of What Firearm and Puppy? magazine. But think of it this way: it costs only 12 grand more than the same-engined V8 F-Type, has several more seats and approximately a million times the bootspace. A 92-grand XJ? It's a bargain.
5000cc, V8 petrol, 542bhp, 502lb ft, 24.4mpg, 270g/km CO2, 0-62 in 4.4secs, 174mph, 1870kg, £92,370
Enormous, thirsty, deliriously fast, arguably pointless. And therefore possibly our very favourite fast Jag.