This is getting silly. We joined the autobahn at 55mph and all I did was straighten my right foot. There was a muted whoosh followed by acceleration so explosive that in barely any time at all it was cut dead at precisely 155mph. It has already taken you longer to read this paragraph.

We would have reached 200mph were it not for the sensible chip in all German saloons that prevents them exceeding 155mph. The truth is this chip is often used as a marketing gimmick to hint at untold though often fictitious potential. Drive the new Mercedes-Benz S 600 at 155mph, however, and you'll find it's just getting into its stride. I suspect I'm meant to think that's fantastic but, for reasons I'll come to shortly, I found it rather irksome.

First some hard facts. This latest Mercedes has a new 5.5-litre V12 boosted by two turbochargers that liberate 500 horsepower, 100bhp more than a Ferrari 360 Modena. It also has more than twice the Ferrari's torque and it is torque you feel when you accelerate. Depending on your mood it can seem heroically, comically and pointlessly fast. No saloon I have driven, not even Audi's new RS6, gathers speed like this above 100mph. My question is: why?

The S-class Mercedes has been the world's greatest luxury car for many years and this one, with minor cosmetic updates, seeks to tighten its grip on the title. The whole range is fitted with new bumpers and clear headlamps, while better cabin materials smooth out the wrinkles in the four-year-old design. A new safety system, Pre-safe, can even take an educated stab at whether you're about to crash. If so, it will tighten your seat belt, move your seat and even shut the sunroof in anticipation.

Mechanical changes include more power and lower fuel consumption for the diesel S 320 CDI and a new 3.7 litre petrol engine for the S 350, to replace the S 320. But the big news is that bi-turbo V12: by giving it 500bhp Mercedes has raised the ceiling of saloon car power by 100bhp and thrown down the gauntlet to Audi, BMW, Lexus and Jaguar. If I were them, I'd leave it where it is.

The point is this: the S 600 is a luxury saloon and at £87,580 a damned expensive one, yet it is not substantially more luxurious than an S 350 costing £37,770 less. If it's more refined at speed these ears can't detect it, while its softer suspension serves up a ride inferior to its massively cheaper stablemates. To me the notion of asking that much more money for a luxury car that offers no more real luxury is fundamentally flawed.

The S 600's purpose is Jonesmanship, and those desperate to prove theirs is bigger than yours have been around since before the automobile. However, you'd need Blofeld's thirst for power to conclude that, of all options, half a thousand horsepower is really what's required to improve an executive express.

I'd have been so much more impressed if its performance were no better than the still rapid S 500 but it had set new standards of sybaritism. Instead Mercedes's complacency has allowed its rivals to get closer than ever. The BMW 7-series is now at least as refined, the Lexus LS430 has a fractionally superior ride, while the Audi A8 has moved within arm's length of the big Benz. A chance to put an entire range beyond the grasp of the opposition has been sacrificed largely for some cosmetic surgery and one megalomaniac engine.

Do not misunderstand me. Power has its place and were this a sporting car I'd have no quarrel. But it's not, and while I appreciate that a monied few will slaver at the prospect, it is in fact the least satisfactory of the range.

Why should Mercedes care? Globally the S-class outsells all its rivals combined and nothing from the opposition looks likely to change that soon. Besides, the S-class still offers the best range of luxury saloons in the world. But if you're after one, do yourself a favour: save some money and ignore the extraordinary, outrageous irrelevance at its top.

Vital statistics

Model Mercedes-Benz S 600L
Engine type 12 cylinders, 5513cc
Power 500bhp @ 5000rpm
Torque 590 lb ft @ 1800rpm
Transmission Five-speed automatic
Suspension (front) double wishbones air springs; (rear) multi- link axle, air springs
Tyres 275/45 ZR18 Fuel (combined) 19.1mpg
Acceleration 0 to 60mph: 6.8sec
Top speed 155mph (limited)
Insurance 20
Price £87,580
Verdict Buy an S 350 and a Porsche Boxster S with the change