Lexus GS 450h

Price: from £38,015 to £47,135
Engine: Hybrid powertrain of 3,456cc V6 petrol coupled with 197 bhp electric motor
Transmission: Automatic/ six-speed sequential
Performance: 155mph; 0 to 60 in 5.9 secs; 35.8 mpg.
CO2: 186g/km

They're not modest, the folks at Lexus, Toyota's prestige brand. On the recent European launch of their new hybrid saloon, the GS 450h, they made remarkable claims about their car. "Lexus engineers achieved levels of NOx and HC emissions that, combined, are lower than any other combustion engine on the market." Well, can it really be lower in any meaningful way than those teeny-weeny diesels that Citroën makes, I wonder?

"The new Lexus is comprehensively equipped with the most technologically advanced pre-emptive, active and passive safety systems available today."

Not sure about that, either. The Mercedes-Benz S-Class seems to have the edge there, but once you're into double figures on air bags, who's counting?

"The world's most technologically advanced drivetrain." Well, maybe the one in the Lexus RX hybrid is an even greater accomplishment, having overcome the additional complexity of four-wheel drive. Then again, Ford has also done that in the States with its Escape hybrid SUV, but maybe that's not so clever.

If you're exposed to Lexus propaganda for too long you start to wonder what God might have found himself doing if the Lexus engineers had been around during the Creation. Getting the teas in, probably.

Despite the hyperbole, you may already know this Lexus not as the most fabby car on the planet, but the one that David Cameron decided to save the planet in. The Lexus 450h is the official car of the leader of the Opposition, or rather, the car that the Tory leader's chauffeur, paperwork and clothes travel in behind Mr Cameron when he's riding his bicycle and being all green for the media. You may remember when this particular bit of political stuntery was exposed. Whatever you made of it, the most welcome outcome of all the fuss is that the ideal of greener motoring and awareness of the alternative technologies already on the market have gained wider currency.

So now we've all heard of the hybrid Lexus 450h, what are we to make of it? Like all Lexus cars, it is a superbly built, comfortable, fast cruiser, full of fun technology (like the colour reversing-camera). To be sure, this is a very clever machine. As with all hybrids, it uses power otherwise wasted while braking and decelerating to charge batteries that can drive an electric motor. That motor can pull the Lexus along on its own for very short periods at very low speeds. More often, it helps the petrol engine out, cutting its efforts, improving performance and fuel consumption.

Perhaps the most impressive thing about it is the strength of that electric motor, which has a maximum power output of 197bhp - on its own about the same as that found in the V6 Jaguar X-type's petrol unit, say. Coupled, as it is in this hybrid, with the Lexus's smooth 3.5-litre petrol V6 and its 292bhp, you have a car that has very capable performance indeed - about the same as a 4.5-litre petrol V8, for the sake of argument, which is why Lexus has given it the model designation 450.

Electric motors also deliver their power smoothly, and Lexus has added a sort of mini-auto gearbox within the electric unit that helps it deliver even more low-down punch. You may not have thought of a hybrid as a performance machine - and it isn't perfect - but the electric motor and petrol engine makes for an intriguing combination.

The downside is the constantly variable transmission (CVT) gearbox, which has a whiney, thrashy quality, and the extra weight of the batteries, which will take the edge off performance. Having said that, this Lexus is equipped with an override that turns the CVT box into a six-speed sequential one. Maybe those Lexus engineers are almost as clever as they say they are. There are various buttons and gearbox settings you can select that push your Lexus along the spectrum from slightly wallowy vaguely green limo to full-on sports saloon. If you choose to exploit all of this machine's performance, you'll find it getting you to 60mph in under six seconds, from 50mph to 75mph in 4.7 seconds and onwards to a (governed) 155mph.

All that, and an official average of 35.8mpg. Emissions of CO2 and other noxious substances are very low for a car of this size and bulk. The official verdict is that this Lexus pumps out 186g of CO2 every kilometre. That's about the same as a Mondeo, but it's not that far astray of the equivalents for the Lexus's main opposition, the German diesels.

So the BMW 530d manages 179g/km and the Mercedes-Benz E320 a credible 194g/km. True, being a petrol rather than a diesel machine, the Lexus has the edge on particulate pollution, and it is way better than its petrol counterparts, but it's not that great either. A diesel hybrid might be a different matter, but given that Lexus is aiming at the US market, and the US doesn't love diesel, I doubt we'll see one soon.

Mr Cameron's stated reason for passing on the offer of the Lexus's smaller brother, the Toyota Prius, was that there wasn't enough room in it. Well, when he opens the boot of his Lexus he's going to get a nasty surprise. There's not much room in there for anything more than a change of boxers and all his party's policy documents, ie, not very much at all.

You can squeeze in a couple of suitcases, but the hatchback Prius is a more capable carrier. Its boot holds 408 litres, against the Lexus 450h's 286. The Prius is narrower than the Lexus, and there is less legroom but there isn't that much in it in the cabin either.

Whereas the Prius was always designed as a hybrid green machine (aerodynamic shape, thoughtful mechanical layout) the Lexus is basically a conversion of an existing conventional sports saloon. For all their cleverness, the Lexus engineers haven't been able to create space where there was none. So they're not that wonderful.

The rivals

Mercedes-Benz E320 CDI Sport £37,740

Europe's favourite exec saloon, but it has lost ground in the States to Lexus, presumably because Americans are less snobby about badges. There's a mildly facelifted and safer version out now.

BMW 535d SE £37,790

Interior ambience and mechanical competence highly impressive. Fine diesel combining excellent performance, economy and low CO2. Downsides are, arguably, styling and the iDrive system.

Citroën C6 2.7 HDI Exclusive £37,850

Imposing and luxurious, in truth it won't divert that many potential Lexus customers, who presumably rate reliability above all. However, it's an interesting alternative to the usual suspects.