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Fuel economy: real-world vs. official figures

One of the biggest complaints drivers have about their cars is poor fuel economy, or more specifically that the car uses a lot more petrol or diesel than the manufacturer’s figures suggest. The Car Expert is here to explain why.

Every new car in the UK has three official fuel consumption (fuel economy) ratings – Urban, Extra-Urban and Combined. Urban is supposed to represent city driving, Extra-Urban represents country roads and motorways, and Combined represents an overall figure.  But most motorists will tell you that they are unable to get anywhere near their car’s official fuel economy figures, and it usually drives them mad.

(Update – October 2015: this topic has been thrust back into the limelight in recent weeks as a result of discoveries that the Volkswagen Group has been cheating its emissions tests. The controversy has led to many people demanding the testing processes for fuel economy and emissions to be completely overhauled.)

How is fuel economy calculated?

A car’s “official” fuel economy figures are calculated to a uniform EU standard, using a carefully-controlled laboratory testing program – known as the New European Driving Cycle. This means that every car on sale in Europe can be compared using the same tests, so you can compare the relative fuel economy of Car A with Car B. The current test has been in place since 1999, which means you can compare a brand new car with one that is several years old.

Lab testing is an important part of the process, as it ensures that results are consistent across different manufacturers and models. It also eliminates variables such as traffic conditions and different driving styles. However, the current testing does have its drawbacks.

Firstly, the manufacturers are able to conduct their own testing, using vehicles specifically chosen for the task. This immediately leads to accusations of car companies ‘fiddling’ the results in their favour. The nominated government agency (in the UK, it’s the Vehicle Certification Agency) does “inspect the test laboratories and witness some tests being carried out”, but this is definitely not the same thing as the test being conducted on random vehicles by an independent body.

Secondly, the test routines are relatively simple and do not tax the car greatly, meaning the results will inevitably be very favourable. There are no complications like hills, headwinds, traffic, different drivers with different driving styles, weather variables, luggage, under-inflated tyres, and other factors which will affect fuel economy. The results will always be a best-case scenario.

Thirdly, car manufacturers are very good at designing their cars to excel at the straightforward government fuel consumption tests. Modern on-board computers control every aspect of how your car performs, and those computers are programmed to recognise when the car is being lab-tested. If the car detects that it is being tested in a laboratory setting, all the systems are optimised for maximum fuel economy, so the car will get the best-possible results in the official fuel economy tests – even if they are not representative of the car’s normal operating environment. Sneaky, huh?
(Update: The current emissions scandal involving the Volkswagen Group goes above and beyond this, with additional software installed which specifically switches off emissions equipment in normal driving, and only switches it on when the car detects it is being lab-tested. Although these tests were not measuring fuel economy, the systems concerned do affect the car’s economy and performance – which is why the software switches them off most of the time.)

The net result is that the official fuel economy figures will almost certainly be considerably better than what you can expect to achieve in the real world (unless you do all your driving downhill with a tailwind). So why are the tests not just made tougher? Well, mainly because that would mean that you wouldn’t be able to compare a car tested under the new method with a car tested under the old method, and re-testing every car that has been on the market since 1999 (when the current test was devised) is not economically feasible. There is also not a lot of interest from the manufacturers, as tougher tests would make their new cars appear less economical than their old cars, even if this is not really true.

Various other people or bodies have had a crack at providing ‘more realistic’ fuel economy figures for cars, such as WhatCar? magazine’s ‘True MPG’ program. Although the fuel economy numbers they provide may be more ‘realistic’ (ie – thirstier) than the official tests by driving on ‘real roads’ and carrying more weight, they cannot match the consistency of laboratory testing, and introduce all-new margins of error when estimating what you can expect a car’s fuel economy to be for your circumstances.

The best solution is to come up with a new laboratory test that is more taxing for the cars, so that the results are more representative of what driers in the real world can expect. Although this will mean the numbers will no longer be comparable to the ‘old’ figures, it would be relatively easy to test a reasonable number of vehicles on both the ‘old’ and ‘new’ test to give an approximation of the average difference. In addition, the figures should be calculated by an independent body, not by the manufacturers themselves. (Update: maybe the fines Volkswagen will inevitably be given can pay for this body to be set up…)

So how do I know what sort of fuel economy I can expect?

From a buyer’s perspective, the best way to look at any fuel economy claims is to view them simply as a guide to a car’s relative fuel economy rather than its absolute fuel economy. If your current car is officially rated at 30mpg but you only achieve 24mpg in your own driving, then you can expect a reasonably similar ratio when you’re looking at a new car which is officially rated at 40mpg (so, expect to get roughly 32mpg).

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The Car Expert compares real-world fuel economy to official fuel consumption figures

This article was originally published in May 2012, and has been updated in October 2015 with additional comments relating to the current emissions scandal concerning the Volkswagen Group and general public debate about fuel economy testing.

Stuart Masson
Stuart Massonhttps://www.thecarexpert.co.uk/
Stuart is the Editorial Director of our suite of sites: The Car Expert, The Van Expert and The Truck Expert. Originally from Australia, Stuart has had a passion for cars and the automotive industry for over thirty years. He spent a decade in automotive retail, and now works tirelessly to help car buyers by providing independent and impartial advice.
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