Not Fit for Purpose: MPG figures

MPG Figures

MPG is in the news this week. The annual MPG Marathon has been taking place, as a fuel-miser exercise in excessive frugality. By driving in an extreme manner, rather than at the pace and urgency of normal traffic, feather-footed eco-obsessives manage to record some pretty amazing figures.

This year’s event started near Oxford and pootled through the Cotswolds, South Gloucestershire, Wiltshire and Hampshire. The winner usually tops 100 mpg, using techniques such as miserly acceleration, minimal braking, slip-streaming bigger vehicles to reduce drag, switching off fuel-hungry electrics like air conditioning, crawling up hills and free-wheeling down them.

It’s far removed from the way most of us drive. Cruise at 70 mph on the motorway? Much too thirsty a habit. Accelerate away from the lights? You might was well chuck handfuls of £ coins out of the car window. Want some cooling air from the vents on a warm day, or a bit of comfort from the car heater on a cold one? Tough, go without.

The closest most of us get to an mpg marathon is a long motorway haul with inevitable fuel stops at the Services. Those of us who like to keep an eye on fuel consumption are only too unhappily aware of how economy fares in normal driving. The kind of mpg figure we regularly achieve is typically well below the official government fuel consumption figure for that particular car, never mind any hope of exceeding it.


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The combined average fuel figures as notoriously optimistic, and I would argue are simply not fit for purpose. They are a combination of the urban figure and the so-called ‘extra-urban’. These are the result of test cycles the car is put through,  driven by a skilled driver on a ‘rolling road’ apparatus. The driver has to carefully follow a pre-set line on a moving chart, which is supposed to replicate typical driving patterns. It’s a laboratory method of producing an mpg figure that is supposedly average for the same car driven on normal roads.

The figures have long been criticised as unrealistic, and this week there is fresh evidence of how inaccurate they are. Professional Driver magazine, the respected mouthpiece for the private hire and chauffeur industry, has compared the economy achieved for 75 cars tested in real-world driving conditions with their official fuel figures.

Result: not one car in normal driving matched the manufacturers’ quoted combined mpg figures. The average overstatement was just over 37 per cent, with some cars having an ‘official’ figure as much as 74 per cent above the real-world result.

This comment from Mark Bursa, editor of Professional Driver, is interesting. He said: “While this is not as serious as the manipulation of the US pollution tests by the Volkswagen group, it does highlight the fact that the manufacturers’ quoted mpg figures in every case overstate the fuel economy you might expect to get from your car.” It’s shockingly true.

Because there is a direct link between fuel economy and CO2 emissions, it also means that all the cars tested by the magazine are inevitably emitting more carbon dioxide than the official figures show. That’s bad news for the environment.

So it’s well past time that the official MPG figures were radically overhauled, to give us all a much more reliable gauge of what economy to expect of a car we might buy. There is a downside for us drivers, though, if more realistic economy and emissions figures were used.  Cars with poorer economy and higher CO2 attract more tax. Sometimes you just can’t win.




Sue Baker is a seasoned motoring journalist with a love of all things automotive.

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