The secrets behind official mpg v real life mpg

October 6, 2006 By Green Car Guide

official mpg v real life mpg

There have been numerous articles in the motoring press about differences between ‘official’ miles per gallon figures and what you can expect in real life . Here we reveal the secrets behind the figures, and provide a new set of tests that provide a more accurate guide to the mpg you can expect, from one of the UK’s top experts on your car’s fuel consumption.

Peter de Nayer formerly worked for the AA, where for over thirty years he was a Technical Researcher and Writer, testing cars, accessories and economy devices for the AA’s ‘Drive’ magazine and its website.

Peter personally developed alternative fuel consumption tests which, compared to the ‘official tests’, more accurately demonstrated the differences that arise, in the same car, when subjected to different driving styles and operating conditions.

The results demonstrate that how and where a car is driven has a dramatic impact on its consumption.

The facts are that most official figures are hard to attain in real-life driving.

How ‘official’ fuel tests are conducted

The first surprise to most people may be that the car makers themselves, rather than some independent body, run the tests, albeit to a brief specified by the appropriate department of transport.

The tests themselves are conducted on a ‘Rolling Road’ dynamometer and have two parts. The first part consists of 2.5 miles of accelerating, slowing down and idling in the lower three gears (unless it’s an automatic), after a cold start, at an ambient temperature that we expect to experience on a English summer day. The maximum speed is 30mph and the average 12mph.

Following on immediately from this, the second figure is derived over a further 4.3 miles driving in all gears, accelerating very gently up to a maximum of 75mph, and averaging 39mph. The total fuel consumed in both tests, divided into the total 6.8 miles, gives the combined result.

Peter views this test as being too simplistic and therefore not accurately reflecting the driving of the average motorist. “It’s not that the method of measurement is crude – in fact the laboratory testing method works on the inherently accurate basis of assessing the amount of Carbon Dioxide and other gasses emitted by the car’s exhaust. From this, the researchers are able to calculate both the CO2 figure as well as the MPG ones that you see published. It’s just that the tests themselves do not reflect enough of the vagaries of everyday driving conditions.”

How Peter de Nayer’s fuel economy tests are conducted

With the environmental performance of a car being an increasingly important base for motoring taxes, due to current environmental concerns as well as spiralling fuel costs, independent and realistic assessment of car fuel consumption is more important than it has ever been. Over the last few years, Peter De Nayer has sought to achieve more realistic figures and he has developed a set of five test cycles that have a broader spectrum of driving styles and operating conditions. As there are five tests rather than two, the resultant mid-range calculated average figure is claimed to be more trustworthy for the average owner who does a bit of everything in his or her motoring.

His five fuel consumption test cycles – urban, suburban, motorway, brisk driving and gentle driving – treat the cars to real-life operating conditions, with cold starts from a realistic (for the UK) 10 degrees C included in the test cycles.

These are road-based, with corrected instrumentation, simulated traffic halts and fixed rates of acceleration/deceleration not dictated by other traffic. The routes are circular and the tests are not conducted in the wet, or at significant wind speeds, and ancillary electrical equipment including air conditioning is off.

Fuel measurement is normally by a meter that is inserted in the supply line – not by brim-to-brim measurement that is unreliable over shorter distances, and Peter makes the interesting statement that car trip computers can seldom be relied on – something that Green-Car-Guide would agree with.

In practice, the calculated average consumption is arrived at by using 10% urban, 15% suburban, and 25% each for the other three cycles.

The result of all this attention to detail is a repeatability better than 3 per cent. The results are rounded to the nearest 0.5mpg or 0.1litre/100km.

The Results

The results show that some of the official figures average out to be very close to de Nayer’s results, whereas others are wildly adrift.

Even more significant, however, is that the five test cycles show that even without being subjected to boy-racer driving, the same car can nearly double (or halve) its consumption, depending on how and where it is driven. The clear outcome is that for any car, there is no such thing as a single mpg figure that every owner will attain; lots of cold starts, or high speed cruising in a small car, will get through a lot of fuel.

Perhaps not surprisingly if a hatchback is compared with its identically-engined MPV equivalent (eg VW Golf v Touran) the MPV will always prove thirstier – and a similar-sized SUV, with four-wheel-drive, even more so.

An automatic will use more fuel than a manual, although the additional losses are not so great in larger more powerful cars than they are in smaller ones with smaller engines.

More subtle outcomes include that larger automatics are more economical on the motorway than when they are driven at half the speed around the lanes, whereas smaller manual cars respond the other way round.

The official figures give us some way of comparing different vehicles. However, if we are serious about the environmental impact of our cars and our use of them, we need a more informative set of figures. Transferring the responsibility of running the tests to a fully independent testing agency who would run a variety of tests like de Nayer’s would improve their credibility.

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