Road Test: Porsche Panamera 3.0 V6 S Hybrid 4dr Tiptronic S

Bang for yer gramme. Not a widely recognised measurement in the automotive world at present, but in the next few years it might become one of the most important. In other words, how much horsepower do you get for each gramme of CO2 your car emits?

Some examples. The Toyota Prius produces 98bhp and emits 89g/km of CO2. Ergo, 1.1 bangs per gramme. The Vauxhall VXR8 we tested last month blurts out 425bhp but coughs up a whopping 320g of CO2: 1.33bpg.

And the new Panamera hybrid? An unprecedented 2.35 bangs per gramme. With a 45bhp electric motor working alongside the 329bhp V6, the cleanest Porsche in history produces a maximum 374bhp, but emits just 159g/km of CO2. Which means a £155 tax disc... and in a 168mph GT, the fastest hybrid in history. Of course, when the Panamera is generating 374bhp, it won't be emitting 159g/km of CO2, but that's hardly the point, is it? There's a game to be played with the taxman, and the Panamera is playing it admirably.

The Hybridamera uses the same basic system as last year's Cayenne hybrid. It can run on electric power alone to 53mph, or shut down the engine and ‘sail' along at speeds up to 103mph, or use petrol and electric power in tandem. Unlike the Cayenne, the Panamera hybrid actually works.

In the big SUV, pressure on the throttle or brake bore almost no relation to the car's decision to go faster or slower. But, thanks to a handful of hardware changes and major software overhaul, the Panamera's hybrid system does its work with impressive anonymity.

OK, OK. A couple of times, the KERS system clung onto the brakes for a moment too long. Once or twice, on kickdown, the Panamera's hybrid cerebellum took a few milliseconds too long to gather up and deploy all the power. But, for the vast, vast majority of the time, the system works flawlessly.

Be aware: the only way to achieve that 159g/km figure is by speccing the 19-inch wheels, which achieve a lower rolling resistance than the 18s - weird, eh? This is a pity, as the smaller wheels give a more compliant ride, but by and large the Panamera still strikes an impressive balance between cornerability and long-distance cruising, the weight of the 70kg nickel hydride battery pack under the rear axle virtually impossible to discern.

It will remain a rare sight in the UK, where the cheaper diesel will make more sense for the fuel-bill-conscious. But in countries with bigger tax breaks for hybrids - the US - it'll succeed as that rarest of things: a hybrid that's an improvement not a compromise. Grammes successfully banged.