Road testing cars does confer certain benefits. The ability to receive a vehicle brim full of fuel and return it a week later running on fumes is one and this tends to engender a rather detached view when it comes to fuel economy and emissions. What it doesn't divorce us from is the disapproving looks from many drivers when a big, luxury 4x4 hoves into view. The Lexus RX has long been one of the less overblown luxury 4x4s and when fitted with a hybrid engine, it's a truck with an environmental conscience.

Of course there are some who feel no guilt about depleting the fossil fuel reserve but even these people's interest could well be piqued by a few of the other fringe benefits of the RX400h. After all, even lazy, selfish people don't want to spend their time queuing in fuel stations. The fuel economy and range of the RX400h means that it'll average around 450 miles on a single fill up. On unfamiliar roads or in freezing weather, that sort of convenience is a real bonus. Lexus quote a combined fuel economy figure of 34.9mpg, by far the best of any petrol-powered luxury 4x4.

Prices start at £36,415, with plusher SR, SE and SE-L variants offered respectively at £37,500, £40,310 and £45,280. That's a premium of just under £4,500 over a standard petrol-powered RX350. This price difference can be explained to a certain extent by the fact that the RX400h is more powerful than its petrol counterparts, generating a healthy 272bhp. This results in a top speed of 124mph and sprinting capability that can shame some hot hatches. In fact, this heavy 4x4 has jet heels, getting to 60mph in just 7.4 seconds. By contrast, a UK specification RX350 will stop the clock at 7.8 seconds, so you're buying a markedly quicker vehicle, but how does this hybrid technology work?

"A lean, green, thrilling machine? Almost?."

The 3.3-litre V6 petrol engine under the bonnet isn't anything ground breaking, but Lexus has also engineered a pair of high-torque electric motors positioned front and rear that can either assist the petrol engine during acceleration or replace it altogether at start up or low to mid-range speeds. Moreover, during deceleration and under braking, the engine switches off and both the electric motors act as high-output generators, driven by all four wheels. This regenerative braking system optimises energy management by recovering kinetic energy that would normally be lost as heat as electrical energy for storage in the high performance battery. The battery power level is constantly maintained by the engine-driven generator, which means there is no need for the system to be recharged from an external power source.

The batteries themselves are zero maintenance items guaranteed for 100,000 miles and are a lot more compact than you'd imagine. This sort of drive system is similar to that pioneered in Toyota's Prius but Lexus have finessed certain elements to suit the character of a luxury 4x4. Electrical power enhancements to the new Hybrid Synergy Drive include the use of a boost converter to increase generating capacity. A new, high speed front motor operates at twice the speed and delivers more than double the power of the one used in the Prius. The addition of a rear motor facilitates four-wheel drive and a new high power nickel-metal hydride battery is used. The rearmost motor can deliver a 68bhp boost to the rear wheels when you need maximum acceleration or when the computers detect front wheel slippage. The RX has never been much of a mud plugger but on snowy roads and wet grass it should be more than adequate.

At all speeds, the Hybrid Synergy Drive system monitors itself for optimum performance and fuel efficiency with minimum emissions and adapts accordingly to run the electric motors in isolation, the engine in isolation, or a combination of both. The driver can see which power sources are being used by means of an instrument readout which replaces the usual rev counter. In versions with satellite navigation, power flow and battery status are shown on a seven-inch colour monitor.

"This is the first time that hybrid technology has been applied to a premium segment car," said Stuart McCullough, Director of Lexus Europe, "and we believe it will surprise many people in terms of what this technology holds for cars of the future, and particularly for Lexus. Hybrid technology represents precisely the type of driving characteristics that are the hallmark of Lexus engineering. Our powertrain is incredibly smooth, powerful and refined, yet gives a highly engaging and rewarding driving experience. And this is only the start of our involvement with hybrid solutions. Lexus is placing an enormous significance on the future of this drivetrain technology."

Ignore the hi-tech trickery under the skin and the RX400h shares all of the normal RX attributes. As well as the usual set of twin front and side airbags, it features curtain shield bags at the sides and knee airbags for the driver. These bags help to reduce injury caused by contact with the unyielding steering column, but tests have shown an additional benefit. Knee bags also reduce head and chest injuries. How? In a usual front end collision, the legs come into contact with the lower dashboard and steering column. Rarely contacting square on, this causes the pelvis to cant forward and begins a twist in the driver's body that can prevent the airbag in the steering wheel doing its job to optimum effect. Knee bags nip this in the bud, making for a safer car.

Quicker, more economical, cleaner and more convenient than the RX350, the RX400h looks like a winner. With pricing that's competitive, it's almost a no-brainer and the lack of a diesel engine in the RX line up may no longer be an issue. This technology is maturing fast and will soon be in mainstream cars. The future isn't looking so bad after all.