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But again, I'm afraid, it's all a bit theoretical. In its normal configuration, the Bugatti Veyron does a piffling 370kmh, or 229.9069mph.

A downforce of 350kg sees to that, since down-force, like lift, can only be achieved at the cost of induced aerodynamic drag. For a Vmax run, a second, special key must be inserted.

This changes the parameters of the computers controlling the variable rear wing and the underbody diffusers, and reduces downforce to just 50kg. This must only be done on a road with no bends, a Bugatti engineer tells me, earnestly.

But, inevitably, there are still issues. The low rolling resistance of the specially developed Michelin tyres - 365mm section at the back, and bigger than even the Countach's fabled back boots - are said to contribute some 15kmh to the top speed quest.

But they cannot do it for long. Maybe 15 minutes. After that, they might disintegrate, and they cost £1,200 each.

Then again, at maximum speed the Veyron is consuming a litre per kilometre, so with luck you'll run out of petrol first. And at 407.5kmh, think how far you'll go in 15 minutes. I'll tell you - it's 63.302067 miles.

However, you have to go to Australia's Nullabor Plain to find a dead straight road that long, and at that speed a stray kangaroo is going to attain the permanence of York Minster.

Even a 0-400kmh-0 run needs a 2.8-mile stretch. Empty, ideally. And people think my Boxster is unusable in the real world.

But here's the real bombshell, and which I've deliberately saved until now. And I'm not quite sure how to say it. Um. It doesn't actually feel that quick.

Not like a Noble M12 or a Ferrari F430. Third-gear acceleration in those cars is like being inside the football at kick-off. But in the Veyron it's more of a hefty shove, more indomitable than shocking.

It's a torquey car with a relatively low redline of 6,500rpm that thunders rather than screams.

A weight of almost two tons doesn't help and neither does turbocharging, which, no matter how many of them you've got, doesn't give the instant crack of a normal engine.