LEXUS this week rammed home its claim of being the most environmentally proactive luxury-car brand with the launch of the second luxury hybrid to be seen in Australia so far this year, the RX400h SUV.

And the new upmarket hybrid is not just a token model in the Lexus range. With average monthly sales targeted at about 60 vehicles, it will represent a significant chunk of RX activity.

The prestige Lexus SUV is sitting at about 200 sales a month right now, which year-to-date is down 37 vehicles on August 2005 even if the overall YTD tally of 1721 vehicles is up on last year, by just 1.7 percent.

Still, this is enough to place the RX as the top-selling prestige SUV in Australia, just ahead of the Mercedes-Benz M-class and leading Nissan’s Murano and BMW’s X5 by about 200 vehicles.

The RX400h is a massively bold move by Lexus, seen by the company not only as a vehicle that significantly stamps its green card, but also as an alternative to the turbo-diesels to which SUV buyers are flocking – to the tune of 36 per cent of total luxury SUV sales compared with 27 per cent five years ago.

Following closely on the heels of the Lexus GS450h sedan, the RX400h brings more than mere hybrid technology too. It also uses it to deliver an all-wheel drive system that is unique to the market in that it drives the rear wheels exclusively with battery power.

While the rest of the system is not that dissimilar to what we have become accustomed to in Toyota’s front-drive Prius, the RX400h adds part-time rear-wheel drive in a system dubbed E-Four.

This is quite a different drive arrangement to that used in the regular full-time AWD RX and is just part of a drastically changed driveline that also includes, for the first time in the RX, electric power steering.

This has allowed the RX400h to adopt a version of the Vehicle Dynamics Integrated Management (VDiM), first introduced on the current-generation GS430, which adds automatic steering corrections to the usual wheel-braking functions of regular stability control systems.

The RX400h goes a step further by including the ability to adjust power feed between front and rear wheels as extra assistance to the stability control.

But what the new Lexus is all about, really, is the hybrid driveline. It comprises a slightly smaller 3.3-litre version of the RX350’s 3.5-litre transverse petrol V6 producing 155kW at 5600rpm and 288Nm of torque at 4400rpm.

The DOHC 24-valve VVT engine works in conjunction with a 650-volt electric motor to provide a system that, in theory, uses zero-emission electricity to provide power at crawling speeds and brings in the conventional V6 to boost acceleration, assist when cruising and charge the battery when it is running low.

The difference with the RX400h is the second 650-volt electric motor exclusively devoted to driving the back wheels when required. Four-wheel drive vehicles using electric motors have been mooted for some time, but this is the first one we have seen Down Under, and it differs substantially from the full-time AWD three-differential set-up used in the RX350.

Managing the complex interplay of petrol power, electric power, storage and generation of electricity is a clever transmission similar to that used in the Toyota Prius. It manipulates a planetary gearset in a way that provides constantly variable ratios while controlling the flow of power from both the petrol engine and the front electric motor.

As in the Prius, and the GS450h, all this goes unnoticed by the driver other than by a visual display on the instrument panel.

The total power of the drive system is 200kW, yet Lexus claims the average fuel consumption for the RX400h is 8.1L/100km on the ADR81/01 cycle (the regular RX350’s official figure is 11.2L/100km), which the company also says is better than most turbodiesel autos by 14 to 20 per cent.

And the RX400h’s CO2 emissions, at 192g/km, are slightly more than half of some cars with similar-sized petrol engines.

The hybrid driveline does have some disadvantages though: the weight of the RX400h is up by nearly 200kg over the RX350.

Lexus backs up the RX400h’s exceptional frugality and justifies the pricetag with a full complement of safety and luxury equipment.

In addition to the advanced safety gear including the VDiM system, traction control, seven airbags (including a driver’s knee bag) and rearview camera, the RX400h gets full leather trim, dual power front seats, a power sunroof, an MP3-compatible 11-speaker Mark Levinson audio system with six-disc DVD/CD changer, adaptive Xenon headlights, satellite navigation and 18-inch alloy wheels.