There is, it need hardly be said, no excuse for this car whatever. The very idea of a large off-roader with an engine more than twice the usual size, running on petrol rather than diesel, with a combined fuel economy figure on the dark side of 18mpg and a CO2 figure that looks like someone's telephone number, is enough to inspire howls of vengeful rage from the environmentalists. If you were to drive a Grand Cherokee SRT-8 into Central London, it's not beyond the realms of imagination that the Mayor would set fire to you.It's not even as if the thing has any practical application. The press specification sheet ends with the words, "Please note that this vehicle is not designed for off-road use!" in bold, underlined capitals. And it's not difficult to see why.Under the bonnet of this Grand Cherokee lies Chrysler's famous Hemi V8 engine, but not in its usual form. No, in this application (as with other SRTs) it's been enlarged from 5.7 to 6.1 litres, with the result that it produces a maximum of 420bhp. This power is taken via a five-speed automatic gearbox and a full-time on-demand four-wheel drive system to huge 20" wheels on which are mounted low-profile 245/45 tyres.There is nothing here to suggest that taking the SRT-8 on to anything as challenging as a slightly moist field is remotely advisable if you have ambitions of regaining solid ground without being winched out. Clearly the on-roading part of the SUV compromise has become dominant, so what's important here is how the big Jeep performs on tarmac.And the answer is that it does rather well, all things considered. I'm reminded of a colleague's reaction when he first went to a truck race meeting at Donington a few years ago. "It was like watching elephants at Ascot," he said, and the Grand Cherokee SRT-8 has that sort of flavour to it, too. In each case, the surprise element comes not from the fact that the thing can be done at all, but that it is being done unexpectedly well.According to Jeep's own figures, this most unlikely of sports cars has a top speed of 152mph and will accelerate from rest to 62mph in five seconds flat. That's not particularly special for a vehicle with 420bhp, but it's quite something in one which weighs 2750kg and has about the same aerodynamic efficiency that you would get if you stood the Isle of Man on one end and faced it into the prevailing wind.It's impressive stuff, and - better still - it's accompanied by the characteristic boom of a classic American V8, one you can enjoy in equal measure whether you're sitting inside or watching someone else have the fun. What's really impressive, though, is the way Jeep has put all that power into a chassis that doesn't want it and ended up with something that's quite friendly to its driver.The Grand Cherokee has too much weight, too high up for this sort of thing, and to avoid transferring the weight quickly enough to distress the tyres Jeep has kept the suspension quite soft. That leads to a lot of body roll, but the damping has been nicely judged, so you have to go a bit mental before you'll get into serious trouble. And for the same reasons, the ride is far better than you might expect, though don't blame me if your passengers start throwing things at you after you've sent them slamming into the side windows once too often.I liked the SRT-8 more than I thought I would. If you were thinking of buying me one as a Christmas present, though, you can get lost. There may be more pointless vehicles available for sale in this country, but it might take me a while to work out what they are. On the other hand, the very ridiculousness of this monster is probably what will appeal to the handful of people prepared to buy it. Engine 6063 cc, 8 cylinders Power 420 bhp @6000 rpm Torque 420 ib/ft @4800 rpm Transmission 5 speed auto Fuel/CO2 17.2 mpg / 388 g/km Acceleration 0-62mph: 5sec Top speed 152 mph Price From £43395.00 approx Details correct at publication date