What does your car do to the gallon? The answer may surprise you

even-supercharged-jaguars-like-this-xjr-can-play-at-being-fuel-economy-champions

APPARENTLY I have a knack for extracting big numbers from fast cars. Only they’re not the figures you might be expecting.

Take the 22-year-old Jaguar XJR I was lucky enough to be lent a couple of weeks ago. Teaming a 4.0-litre straight six up with an Eaton supercharger means it’ll kick 315bhp through its rear wheels – most of which I used on a glorious day cruising up the A1 and then hoofing around the Yorkshire Dales. Even today it’ll crack 60mph in under 6.5 seconds and theoretically race to 150mph, but the number that impressed everyone when I returned home was the 26mpg I’d somehow suckled out of it.

It’s the same story with an equally ancient Mercedes-Benz S280 I borrowed for a trip to Devon not long ago. You’d think having the a smallish V6 thrashing about beneath the bonnet of something the size of my old student flat would be a fuel economy nightmare, but it very nearly managed 30mpg. 

Yet I’ve never been able to pull off the same trick in what’s meant to be a champion of petrol abstinence – the Ecoboost-engined version’s of Ford’s Focus. On the many occasions I’ve driven them I’ve never bettered 37mpg – a long way short of the 56mpg it’s claimed to be capable of.

Sound familiar? Well, some scientist types noticed that lots of you have the same problem of buying cars and then spending rather more at the pumps than you’d originally expected and decided to do something about it. After driving for what must have felt like an eternity they’ve managed to come up with real world figures for pretty much every car you can buy today – and they make intriguing reading.

Not only was their Focus Ecoboost result virtually identical to mine, but they also failed to match the fuel economy of cars marketed as being some of the nation’s most frugal. The Fiat 500 TwinAir gave them 39mpg, making it in their books less economical than the bigger 1.2-litre engine you can buy in the same car. The Golf Bluemotion was 23mpg short of the 74.3 claimed in Volkswagen’s figures. Don’t think buying a hybrid’s a get-out-of-jail-free card either, because the Prius’ shortfall was virtually identical.

But don’t take my word for it – have a look at the Equa Index website and see how your car does. The one constant factor is that the bigger the engine, the less dramatic the shortfall. Buy a Porsche 911 Carrera S and the shortfall is only 3mpg – and chances are if you’re in one of those you’ll be able to afford the difference.

Obviously I’d be more than happy, Porsche, if you’d like me to put the 911 Carrera S through the Simister fuel economy test that so clearly suited the XJR and S-Class. Just leave the keys on The Champion’s reception desk…

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