Not for the first time in our recent experience, a turbo diesel engine has been fitted to a car which is in nearly all other respects unworthy of it. The two lowest-spec examples of the new four-model Jeep Grand Cherokee range - of which this is the better-equipped - both use the Mercedes-Benz three-litre turbo diesel engine, and by any standards it is a superb unit.So much so, in fact, that there seems little point in specifying a Grand Cherokee with anything else. You might choose the 4.7-litre V8, but it's barely more powerful and does not make the car significantly quicker. The 5.7-litre Hemi V8 (which, like the CRD, is also found in the Chrysler 300C saloon that has impressed us so strongly) is substantially stronger, but again the performance difference isn't dramatic, and with that unit the Grand uses about half as much fuel again on every part of the official economy test.But you wouldn't buy a diesel because they're so noisy, is that it? Think again. Jeep diesels throughout history have made the most phenomenal racket, even when the Jeep people have said that this time it's different, this time they've cracked the refinement problem. They never had before, but they have now. The Grand Cherokee diesel is just astonishingly smooth and subdued in most conditions, and takes on nothing but the most delicate six-cylinder whirr when you ask it to accelerate hard.All of the above should be the prelude to a discussion of how the sense of mechanical refinement complements road manners that redefine the dynamic behaviour of the large off-roader. Sorry. Listening to and feeling the Grand Cherokee in action invoke two outstandingly different responses. You can marvel at the way it both whispers along A-roads while also wobbling along them in a way that would have attracted criticism in an off-roader built in the 1980s.There's a road near these parts which includes a long, plunging left-hand bend. I have gone through this corner in well over half the cars I have tested in the last six years, and I can barely recall one which caused me the same level of concern that the Grand Cherokee did, with the possible and ironic exception of the Cherokee before its suspension was revised recently. I certainly don't remember past versions of the Grand being quite so unsure of themselves.And you don't have to be tackling particularly difficult countryside to feel disappointed with the car. Almost any road will do it - the effect occurs nearly all the time. Why is this, when more than a decade ago Jeep was building off-roaders which tackled tarmac in a way that made them the envy of rival manufacturers?There are plus points. I like the way the new Grand Cherokee looks, having felt that its predecessors were becoming too bulky (and don't get me started on what I think about the smaller, non-Grand Cherokee's appearance). This one manages to look almost sleek in a way you might not believe possible of a car with such chunky lines and sharp angles.Equipment levels are high, and if you want them higher on the CRD you should go for the Limited specification which is standard on the V8s but an option here that takes the price beyond £30,000. The extra equipment includes a tyre pressure monitor, higher-spec alloy wheels (still 7.5x17" though), multi-function door mirrors, rear park assist, dual zone air-conditioning, an auto-dimming interior mirror and leather trim.The Grand is practical, too, with 978 litres of luggage volume when the rear seat is up, 1909 litres when it's down and a towing capacity of over three tonnes. Jeeps have always been pretty good off-road, too, though you wouldn't want to take the Grand over tracks which would give a Discovery pause for thought.But who would do that anyway? The vast majority of Grand Cherokee owners surely want a car that works well on the road, and the CRD - despite that superb engine - simply doesn't. Engine 2987 cc, 6 cylinders Power 215 bhp @4000 rpm Torque 376 ib/ft @1600 rpm Transmission 5 speed auto Fuel/CO2 27.7 mpg / 270 g/km Acceleration 0-62mph: 9sec Top speed 124 mph Price From £31650.00 approx Release date 04/06/2005