Page 242 of the Bible-sized handbook for the Lexus RX450h could not outline the car's mud-plugging status more plainly: "Your vehicle is not designed to be driven off-road."

Which is odd. Because the next few pages of the guide are given over to what precautions the driver might consider when he or she does muck in. Weird.

To paraphrase, it says you can play in the dirt, but with a lot of care. And no going near the water.

The RX, with its V6 petrol engine married to an electric motor up front, with another electric motor driving the rear wheels, effectively has the generating capability of a small power station. All these zappy bits are unavoidably sited low, the motors under the chassis and the batteries under the back seat. So H-two-oh is a big no-no.

River-wading Landcruiser-style would be an enlightening experience. Literally. In fact, the handbook suggests, it'd be prudent to steer clear of big puddles. "Water entering the engine compartment may cause severe damage to the hybrid system," it says. "Water entering the interior may cause the (nickel-metal hydride) battery ... to short circuit."

While potentially the Japanese are overstating the risks - remember, the RX is aimed particularly at Americans (and we all know how litigious they can be) - it'd obviously pay to exercise a degree of caution.

Ultimately, then, you have to weigh up the usefulness of a vehicle with four-wheel-drive looks, and four-wheel-drive technology, but questionable four-wheel-drive ability.

Lexus will say, as everyone does, that a lot cars this high-end category (save the obvious exception of a Range Rover) are perhaps not more adept and, anyway, even those few that can probably never do. It's not what they're bought for, so no problem.

But surely this sidesteps the issue? I recall the marketing bods making a big noise, back when this car launched, about how it was superior to like-sized and priced swank diesel SUVs from German brands, primarily for efficiency and environmental reasons, which we'll get to in a moment.

Having now spent a week with it, I say the RX450h really isn't a rival to the BMW X5, Audi Q7 and Mercedes M-Class. They're not the same at all, because setting aside any operator intentions, the diesels can all cope with soil toil, at least to some degree. Whereas the RX might not.

And let's not even think about extremes. I understand why streams are dangerous, but what about snow? It's nothing more than frozen water, after all. On the Lexus limit for a romp through the white stuff, the handbook is disconcertingly vague. Likewise, a lot of swank SUVs congregate around the boat ramp. Could you dare dunk the RX's rear end at all?

With these issues, it's hard to understand how the hybrid RX can seek to be the future of the 4x4 if it can't even act like one. So that's one question mark against its technology.

Alacrity and economy are not easy bedfellows, yet, according to Lexus, this is really where the RX blows away all rivals, petrol and diesel.

If you follow the badge logic, the RX450h should provide performance equivalent to that of a 4.5-litre V8 rather than the 3.5-litre V6 actually under the bonnet, yet it is also claimed to beat diesels by extracting just 6.4 litres per 100km on the overall cycle.

It all sounds fantastic, but reality suggests you're in for disappointment on both counts.

Performance-wise, it's quick alright, but falls is short of being shove-in-the-back fast. The surge is smooth and relentless, but an independently-timed 0-100kmh of around 8.2 seconds puts it well behind tarmac tearing rock-hoppers like the Range Rover Sport or the BMW X6 V8. What we would say is that this is a refined rush, and pleasingly stepless.

Getting eco means feather-footing it. Given the time during test dedicated to keeping the in-dash economy meter in the green (for good) zone, it was disappointing to see our overall average was still 9.7L/100km. While that's better than the 13.5L/100km average we achieved with the lookalike, but purely conventional, RX350 driven earlier in the year, it's also not superior to the X5 3.0-litre diesel also tested some months back.

What brings the RX back into the picture is that it is, as all Lexus products are, brilliantly fitted out and finished. And, regardless that the drivetrain is perhaps not quite the answer that it is made to be, the technology per se is impressively clever.

The reason why the Lexus is hybrid, rather than diesel (or diesel hybrid), is because politically diesels are frowned upon in Japan (they're banned in Tokyo) and North America is the largest customer base. And, Stateside in particular, it's still all about petrol power.

Had Lexus chosen to restrict the car just to those markets, it would still be a big success. While Japan's new car market overall is presently down by at least 30 percent, petrol-electric sales continue to climb. Nearly all hybrids in Japan have long waiting lists thanks to their great economy, clean image and government incentives. Order a Toyota Prius today, and you won't pick it up until early next year.

The Lexus drivetrain is similar to that in the Prius, but on a grander scale. Similar to the one in the previous RX but larger in capacity, the V6 is more efficient and now runs on an Atkinson Cycle (in basic layman's terms, this means it demands a unique crankshaft design). It's mated to a continuously variable automatic transmission and is assisted by an electric motor. These two between them drive the front wheels.

Meanwhile, at the rear there's a second electric motor which supplies the rear wheels with additional power as needed. Power generated during deceleration or from the engine's excess is stored in a three-block battery pack.

Like all Toyota/Lexus hybrids, power can come from the electric motors only, just the petrol engine or a combination of both. The individual outputs range from the 183kW down to just 50kW, but when all three engine/motors are giving their all, Toyota quotes a maximum power figure (it's not as simple as adding all three together) of 220kW.

Regardless of our uncertainty about how much saving it really does give at the pumps, there's less debate that it does help the climate. A lot of people still misunderstand that the primary function of any hybrid is to produce low C02 emissions.

The RX scores well on the carbon dioxide count. Lexus claims a class-leading 150g/km on the overall cycle. I don't have the specialist equipment to check this, but can testify it rarely - even at cold start - seems to smoke the pipes, so the evidence in this case is more supportive.

Sometimes the engine doesn't contribute at all. Be gentle on the throttle when pulling away and the RX450h will move off under electric power only, and with sufficient juice in the batteries and a light foot it calls on the petrol motor only when speeds exceed 45kmh. It won't stay in pure electric mode for long, ultimately just a kay or two, but it's fun trying to keep it there. Excellent refinement is a Lexus given, but obviously in this mode it must be among the world's quietest cars.

You'd imagine Lexus would have gone all exotic in the body design yet, while the styling is very much out there (again, to meet US tastes), the construction remains orthodox. The body, as before, is a steel monocoque, and while there is MacPherson strut suspension at the front, there are new double wishbones at the rear.

Undoubtedly due to its electric enhancements, the RS450h feels heavier on its feet than the RX350 and it's not as sharp. As with the pure V6, the suspension set up is quite stiff, but it's not what you'd call nimble. Low-speed ride quality also lacks the compliance you'd get from a GS or LS sedan. That's a bit vexing, too.

Not argument about the luxury, particularly with the flagship $133,990 Limited edition on test. The soft leather, in particular, is of a rare quality and while the dash presents an intriguing mix of shapes and textures, the quality of the finish is beyond question.

One thing that demands remedial attention is the location of the auxiliary port for your iPod hookup. Siting it in the bottom of the centre console, beneath two shelf units that have to be removed to gain access, is ... well, incredibly silly.

The computer mouse-style controller for the various cabin functions still seems awkward to use even after second acquaintance and not allowing full access to the sat-nav and Bluetooth phone functionality until the car is stationary will frustrate responsible adults.

A typical SUV seating position has the driver sitting high on soft, flat-cushioned chairs that offer a very broad range of adjustability. Forward visibility is also good, although the clever cameras are welcome for judging the car's bulbous shape in tight parking spots.

The floor of the beautifully-trimmed boot is quite high to accommodate the zappy bits beneath, which limits the overall load-carrying space to 496 litres. While that still affords decent bag room, bulky items will pose a challenge. There is, however, a rear seat with a 40-20-40 split-fold set-up, which is handy for longer items.

The RX appeals as a sharp-looking, well-equipped and superbly-built technology showcase, which certainly sets a commendably high standard for environmental responsibility. It's a big four-by-four that can't be accused of being a oversized drain on a dwindling resource.

What weakens its case is that the reality of the fuel savings is well short of the official figures, it's not a cheap car to buy and, if the handbook is taken seriously, it's not going to fulfil the functions that can be taken for granted in even a high-falutin' conventional diesel sports ute.

What this means to you: Technology leader lags in real-world applications.