Let us, please, get the environmental thing out of the way first. Do not be persuaded by the Lexus marketing machine or by the leader of the opposition that this Lexus GS 450h is an overtly environmentally friendly car. It may have more flattering fuel and CO2 figures than its immediate rivals, but it’s not going to help save the planet any more than ordering a Diet Coke with your cheeseburger is going to make you thin.

Truth is, the Lexus uses more fuel and pumps more CO2 into the atmosphere than a mid-sized saloon like a Vauxhall Vectra. Environmentally friendly is walking, a bicycle or using the internet to work from home. It is not a near two-ton £43,290 luxury saloon. Before this year’s budget brought in a new top rate, it sat in the highest category for vehicle excise duty. It is therefore somewhat curious that it is exempt from London’s congestion charge.

A pointless car for those living outside the capital? Far from it. After spending 700 miles in all conditions at the wheel of one, I felt remarkably well disposed towards this form of petrol-electric hybrid, just not for any of the reasons you might expect.

The first point to make is that the GS 300 on which it is based is a very capable vehicle. Good looking, sporting, ultra-refined and fabulously well built it is a competitive alternative to any similar Audi, BMW or Merc.

The GS 450h uses a conventional 3.5 litre petrol engine developing 292bhp and an electric motor fed from a nickel-metal hydride battery that develops 197bhp. Unfortunately you can’t simply add these two numbers to get a total output because the petrol and electric motors can’t offer peak power at the same time, but its combined output is still an impressive 341bhp. Which means under six seconds for the 0-62mph sprint and a top speed of 155mph. And while this makes it only slightly quicker than a typical rival like Audi’s A6 4.2 quattro, it’ll return an average of 35.8mpg compared with the Audi’s 23.9mpg.

Around town, where the Lexus spends much of its time wafting along on electric power alone, the differences are more pronounced still: the GS manages an astonishing 30.7mpg, the Audi 16.9mpg. And while the Lexus’s CO2 emissions are nothing to shout about in ultimate terms, by fat saloon standards they mean those who use one as a company car will be assessed for tax on 21% of its value, compared with 35% for any comparable rival powered by petrol alone.

But what bald statistics fail to convey is the eye-widening shove the electric motor gives this car at real world speeds. Unlike a petrol engine, which has to be at the top of its rev range before it’ll give maximum power, an electric motor delivers the lot from standstill. When you overtake you feel like a long piece of elastic has been released. The gearbox is continuously variable so there’s no pause while it changes gear, no time when the engine is not operating optimally.

There are, however, a couple of serious downsides. First, to make space for the battery, boot space falls from the scarcely capacious 430 litres of the standard car to just 280 litres, or less than a Renault Clio’s boot. With no facility to fold the back seats forward because there’s a socking great battery in the way, that’s a serious consideration.

Secondly, the 71-litre fuel tank of the standard GS has been cut back to 65 litres, which means one potential benefit of its consumption — a massive range — has not been fully realised.

But I still found the GS 450h the most convincing alternative to the German mainstream I have experienced. Clearly the hybrid drive offers considerable economy and some emissions benefits but the talent you notice most is how driveable it makes the car, offering such instant urge in all conditions it makes conventional petrol engines seem a little impotent in the lower reaches of their rev ranges and diesels strangled by their inherently narrow powerbands.

To people who have over 40 grand to spend on a car like this, it brings a green tinge to a rather rosy picture.

THE OPPOSITION

Model BMW 535d M Sport £40,420
For Amazing power and performance, fun to drive
Against Not very frugal, stiff ride, odd looks, iDrive operating system

Model Audi A6 4.2 quattro S line £47,170
For Looks, interior, four-wheel-drive system, image
Against Thirsty, unimpressive ride quality, not that much fun to driv