THUNDERING along a fast, bumpy and winding country back road is not a bad time to reflect on how far Jeep has come with its all-new Grand Cherokee.

Imagine, for a moment, the scenario on the same stretch of road in the previous Jeep.

The smaller-than-you-thought Grand Cherokee would have had you acutely aware of its softish suspension via the noticeable heeling-over, and the cushy seats would have been sure to eventually leave you with some discomfort if the trip persisted for more than a few hours.

This putting into perspective is significant with the all-new flagship Jeep, because it has undergone a dynamic character change that not only shuts the door on previous perceptions, but also brings it into contention with the best of the current mid-size SUVs.

And nowhere moreso than with the just-launched turbo-diesel CRD model, which marries the all-new body structure and its all-new suspension with a muscly all-alloy, multi-camshaft, multi-valve 3.0-litre V6 straight out of the Mercedes-Benz workshops.

In fact it could be that Jeep now has the most impressive turbo-diesel on the current SUV market – if only because it beat the new Mercedes ML series, which also offers a version of the same engine, into the showrooms by a week or so.

Like each successive turbo-diesel that replaces an earlier design, it astounds with its easy starting, strong torque, fuel economy and exhaust cleanliness.

But the new 3.0-litre V6 adds to this by also eliminating virtually all the diesel noise as well. We thought the new Land Rover Discovery’s new turbo-diesel V6 was quiet – and it is – but the Jeep goes a few silent steps further.

Largely, it seems, because the Benz engine utilizes injection technology that takes auto design almost into the realms of new-age mysticism.

The injection process is astonishingly fast and microscopically accurate thanks to the adoption of crystal-actuated piezo ignition that enables the injectors to pulse up to four times at each ignition cycle. This plays a major part in controlling the ignition process and minimising the normal diesel clatter.

And it works. No diesel springs to mind that equals the Grand Cherokee’s V6 for lack of noise. Even under load, the clattering is reduced to a mere whimper, barely detectable from inside or out.

What else does this new engine do?

Well, it’s pretty strong on torque too, with the highest rating yet for a 3.0-litre turbo-diesel with 510Nm at a mere 1600rpm. Jeep says almost 450Nm is being produced at 1000rpm.

The power is impressive as well, with the sort of output you’d expect from a healthy petrol 3.0-litre at 160kW, produced at a lowly 4000rpm.

Driving the CRD for the first time is to be impressed with the smooth, unruffled nature of the engine. It’s easy to forget there’s a diesel working away under the bonnet.

Through the smooth-shifting five-speed sequential auto it accelerates strongly (faster than the Grand Cherokee’s 4.7-litre V8) and cruises with relaxed ease on the open road.

In no way is it hampered by the Grand Cherokee’s weight increase – around 300kg over the outgoing model – whether it’s moving away from a standing start or surging past slower traffic on the open road. It’s a pretty lively SUV.

Considering all this, it was slightly disappointing to find on our extended introductory drive route that the fuel consumption figure never really approached the official averages claimed by the company.

They indicate a combined figure of 10.2L/00km, but we only managed 12.3L/100km on mostly highway driving interspersed with a bit of slower-speed gravel-road work. Time will tell on that one.

The drive is otherwise pretty much the same as the recently launched petrol versions of the Grand Cherokee.

Although it retains a live rear axle (Jeep says this is because it wanted to retain off-road credentials) the Grand Cherokee feels truly nimble and stable at speed, only disturbed by some occasional rack rattle on fast, rough corners and, sometimes, a bit of sideways rear-end shift indicating there’s more unsprung weight moving around than you’d find in an independent set-up.

But the new variable-ratio rack and pinion steering is very accurate, and nicely weighted to the extent it belies the size and weight of the vehicle.

The brakes, according to Jeep right up there with the best when it comes to stopping power, certainly feel very strong. And the long string of electronic fail-safe systems that are designed to keep the car correctly pointed and firmly on its feet provide a solid sense of security.

Take the Jeep onto a rough bush track and it will demonstrate the traditional focus on off-road ability hasn’t blurred noticeably. There’s a full set of low-range gears, the effective Quadra-Drive II system that will ensure drive is always available somewhere, as well as extended wheel travel to help get you where you want to go.

There is an off-road question mark though. The slight lack of ground clearance measures up at a minimum of 209 centimetres and results in some unexpected underbody scraping.

The Grand Cherokee’s cabin feels pretty nice. Its firm seats contrast with the previous too-soft cushions and there’s a nice, simple ergonomic layout. But the wheel is only adjustable for height, and the large transmission tunnel eats into space for the driver’s left foot.

Vision is generally good, although on tight roads, the hefty driver’s-side A-pillar masks a bit too much of the view around right-hand corners.

Surprisingly, there’s a bit of early Grand Cherokee hangover with tight front door openings that require tall people to duck their heads a little more than normal – although once inside the actual headroom is fine, in front or back.

The rear seat shows us that the Grand Cherokee is not as grand, say, as a Discovery because legroom is good rather than very good, although the lack of a third-row seat means there’s a quite handy load area complete with a removable waterproof tray.

No question, this is the best car Jeep has yet produced.

Combine that with arguably the best turbo-diesel on the market, and the Grand Cherokee CRD deserves a high rating on anyone’s mid-size SUV shopping list.