Porsche 918 Spyder Hybrid

The upcoming 585KW Porsche 918 Spyder can do 325kph, 0-100 in under three seconds and, according to the official figures, emits just 70g/km of CO2 while using just 3.0l/100km. So it’s faster than a Porsche 911 Turbo, more economical than a Prius and emits less carbon than a tiny Peugeot 107.

If this all sounds unbelievable there is a very simple reason. It is unbelievable. It is about as likely as Pastor Maldonado winning the F1 championship for Williams this year after being voted the best driver of the year by F1’s team principals.

Yet, owing to the idiosyncrasies and assumptions of the NEDC (New European Driving Cycle) test on which official emission figures are based, that is precisely what the Porsche 918 Spyder is supposedly capable of.

And the Porsche isn’t the only car to boast fanciful official figures. It’s true of all other cars that use plugs as well as petrol.

Equally astonishing was Jaguar’s proposed C-X75 hybrid supercar. Although now shelved, Jaguar had targeted sub-99g CO2 with plus-320kph. This is about as plausible as Frank Bruno making a heavyweight comeback while simultaneously breaking Mo Farah’s 10 000 metres record. Yet, owing more to the foibles of the figures than to Jaguar’s genius, the new Top Cat would probably have achieve d those targets. ‘Officially’, at any rate.

These official CO2 (and associated fuel usage) figures are important. Companies use them to promote their cars. Draconian new regulations to ensure manufacturers make vehicles that belch less carbon will be based on their data. They really matter.

Of course, we all know they are deeply flawed. We know this because most motorists are unlikely to get within 20% of the official ‘combined’ consumption figures.

Hybrids, in my experience, tend to provide the most disappointment, partly because the official test is done at warm temperatures (between 20 and 30deg C) and cold weather reduces the efficiency of the hybrids’ batteries on which they rely for some of their motive power. Cold or very hot weather also means you’ll need to use the electric-powered heater or air conditioner, increasing electric energy consumption and reducing range.

Plus air conditioning, lights and heated windows are all turned off in the test cycle. Again, this is a blessing to the battery-car brigade. Finally, the test is statistically biased to a high amount of urban driving, where hybrids perform better.

The official test is even less accurate for plug-in hybrids such as the 918 Spyder. The reason is simple: the CO2 emissions from power stations used to charge their batteries are ignored. Plus there are numerous assumptions, each of which flatters a plug-in hybrid’s fuel economy.

The NEDC test assumes a plug-in hybrid starts each journey with a fully charged battery on which it can run electric-only until the charge is depleted. Once the battery is exhausted, it assumes you’ll go no further than 25km on petrol power before charging again – highly unlikely if you’re driving a fast-moving 320kph Porsche. If you don’t charge up as regularly as the test assumes, or drive longer distances between recharging, fuel consumption (and thus carbon emissions) will be much higher.

To highlight the disparity of plug-in versus non-plug-in, the normal hybrid Prius officially emits 89g/km of CO2 while the almost mechanically identical (apart from a power cable) ‘Plug-in’ Prius emits 49g – almost half. Fuel figures are equally disparate: 3.9 vs 2.1 (L/100km)

For the same flawed reason, the electric Nissan Leaf – and all other pure battery EVs – have official CO2 figures of ‘zero’.

It’s all nonsense, of course. It’s particularly silly when it comes to this new breed of high-speed plug-in hybrid supercars (918 Spyder, new ‘Enzo’ Ferrari and no doubt the striking McLaren P1) which supposedly can sip fuel like misers while also playing high-speed ‘Ring meister.

For car buyers, seeking government-sanctioned data to make informed choices, this is all rather confusing. Generally, I advise car buyers to ignore these official figures. To paraphrase Churchill, they are a riddle wrapped in a mystery inside an enigma. (Sadly, their draconian effect on car taxation means total disregard is impossible.)

If you want a super economical car, buy a small one. If you want a big thirsty one, buy an SUV. And if you buy a 320kph Porsche, enjoy the drive. But don’t pretend you’re doing the Earth any favours.