Road Test: Mercedes-Benz G Class G63 5dr Tip Auto

A mate of mine remembers being hauled into the headmaster's study and him intoning gravely: "I'm not angry. Just disappointed. You've let yourself down. Let your parents down. And, most of all, let the school down."

I'm not angry at AMG for building the G63. But I am disappointed. They've let themselves down. Let their wonderful V8 twin-turbo engine down. Let the G-Class down, come to that.

The G is a wonderful off-roader. Solid as a mine truck and unstoppable as a goat. Launched 33 years ago, its styling hasn't changed. Nor have the fundamentals of its axles and girder-built chassis. In fact, it's the nature of them that they can't be changed.

All AMG could do was put on huge tyres (which tramline viciously), bigger brakes (which cause the old bus to snake around), and harder springs and dampers (ouch). But there's plenty of room under the bonnet for the mighty V8. The noise is thunder. Sadly, the performance is not lightning. Physics dictate that even 544bhp struggles when it has to contend with 2,550kg and 0.53Cd.

And yet it does feel fast. Partly due to the noise, partly due to the improbability that something this commercial-looking can raise its nose towards the distance with any kind of conviction. But mostly because it is scarily too fast for its own good.

Cornering and braking really are a shambles. The steering's preposterously approximate. The tyres can't give any purchase against this weight of car, but if they could, it probably wouldn't grip and turn, but topple over instead. So the ESP hits it like a tranquilliser dart to a rhino, bringing the beast to its knees at the first sign of a quickly taken bend.

This G-Class gets a modern satnav, instruments and rather tarty leather seats. But they can't disguise the cramped driving position and hurricane-strength wind noise.

But the AMG G isn't about rational judgments. For proof, there's also a G65. It packs the bi-turbo V12, making 612bhp and 737lb ft. It won't be sold in Britain, but elsewhere it's double the price of the G63 - even though it's just 0.1 of a second quicker to 62mph. That makes it the most expensive Mercedes of all. And yet it's sold out for the next two years...