Fuel economy: real-world vs. official figures

Buying a Car
Volkswagen Jetta TDI 'clean' diesel

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 is the Editor of The Car Expert, which he founded in 2011, and our new sister site The Van Expert. Originally from Australia, Stuart has had a passion for cars and the car 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.

14 Comments

  1. Kate Alderdice

    Very interesting stuff – I spotted a comment you made regarding modern usage of on-board computers in cars – having never thought about computers in that context, beyond Formula 1 or supercar scenarios – I’d be interested to hear how that technology works in an every day car, and how it will develop further.

    Reply
    • Stuart Masson

      Hi Kate. Will add it to the list of future blog topics. Hope you’re still loving the Mini! stuart

  2. Hi Stuart
    From a ‘downunder’ old VW Passat driver. Agree with your comments. My experience with fuel figures for my car is very close to what is specified. The combined figure is 6.7litres/100kms with my urban around 7.2 and extra urban at 5.7 with at least 3 people and luggage.
    Do the tests for ‘down under’ vehicles use the same criteria as the EU?
    When will the UK adopt a uniform measurement system MPG what is that??? (dark ages or from the USA)

    Reply
    • Stuart Masson

      Hi Fred, great to hear from you (Fred bought a Volkswagen Passat from me when I worked at a VW dealer in Australia). I believe that the Australian tests are very similar to the EU tests, although they are probably more suitable for Australian conditions due to the (usually) better flow of traffic and ridiculously low open road speed limits. Also, Volkswagens from the mid-2000s seemed to have official fuel economy figures which correlated quite closely with real-world driving. These days VW seems to have joined the rest of the industry in optimising their engine management systems for the official tests while real-world figures are not really much different. Best wishes, stuart.

    • Hi Stuart. I’m another from “down under”. I have a MY2014 Audi 1.8T Multitronic (that has start/stop) that is quoted at 6.9l/100km urban cycle. I drive mostly in light to medium traffic as I leave early for work. My average so far is 12.2l/100km, which is WAAAAY OvER the official urban cycle. I often see 13 and even 14+ L/100km. really disappointing & I dont “thrash” the car!. best I’ve achieved is low 9s on a late night run with no traffic and very few red lights. By comparison, I had a 2002 Renault Scenic RX4 (small SUV) with a 2.0L naturally aspirated engine & it used in the low-mid 10s. So much for advancement in technology……

    • stuart

      Hi Stephen. How long have you had the car? It can take up to 10,000km or more for it to really run in and achieve it’s ultimate fuel economy numbers. However, it is still rare to hit the claimed numbers, as detailed above.

  3. Agree with what you say about new cars looking better on paper but not doing any better in the real world. I have had Passats as company cars for about 8 years, doing about 50,000 miles a year and changing cars every two years. Every new one is supposed to get much better fuel economy figures than the old one, but in reality they’re never really any better. I’m still getting 40mpg, which is pretty much what I’ve always been getting.

    Reply
  4. I have a real problem with my fuel economy, purchased a brand new Audi A4 Avant 2.0 TDi Quattro Dynamik in an 2012, I’ve so far covered just under 8000 miles, and on average my fuel consumption is only 35.2 MPG what was tracked over 7300 miles!

    To me that is ridiculous values below what’s actually produced on the datasheets when you buy a car. I expected it to be lower, but not THAT much lower!

    I contacted Audi Customer Services who said there’s a 33% tolereance level in place, which means on a combined cycle they can quote 53.4MPG but in read life it can be as low as 35.7MPG, but mines even below that, but they are not willing to look into it as a problem. I visited my local Audi dealership who done a fuel consumption test and it came out to 50.4MPG. They used the Audi DIS in order to collect this information so not exactly an accurate reading, mine average was worked out of fuel filling and mileage covered. It took me 3 attempts to get that reading on the DIS which meant driving the car without engaging the turbo, and coasting down a large hill on the way back to the Audi center! Hardly a worth while test!

    If I had known at time of purchase that it was going to be this bad, I’d have probably purchased the petrol as most petrol owners I’ve spoke to get around the 32-34MPG mark, and petrol is cheaper than diesel!

    I find myself in the position where Audi Customer services don’t really want to know, and have fobbed me off to my dealership, the dealership has no cars they can offer me without putting more large sums of money into it. And a car exchange offer means I’d need to payout £3000 to get another car … which would be another 2.0 TDi !

    Audi’s 2.0 TDI Fuel consumption is a joke, and according to multiple forums I’m not the only one experiencing this! I think this large gap needs to be brought to the attention of consumers, and something real world needs to be offered at time of purchase so your better informed to make a decision rather than just getting quote untrue figures!

    Reply
    • Stuart Masson

      Unfortunately, David, stories like yours are relatively common. Not just with Audi or any particular brand, but across the board. This is one of the reasons that the EU is considering a change to the official government fuel consumption test regimen, so that the official figures become more representative of a majority of real-world drivers.

      Of course, fuel economy can be affected by other factors such as driving style, under-inflated tyres, carrying extra weight in the car (a surprising number of people haul all sorts of things around in their car which add up to a significant amount of weight), roof racks, wider tyres (often found in sports models), and so on. ‘Premium’ diesel fuel may give you slightly better economy than ‘regular’ diesel (although it is more expensive so may not actually save you any money).

      Finally, diesel engines can take 10,000 miles or more to truly run in, so it is possible that you will still see some slight improvement on your current figures, although it wouldn’t be a lot.

      Some, all or none of that may help you in your current situation, but keep pestering Audi if you feel there is a problem with the car. Also make sure you keep track of your fuel spending and mileage, and keep track of your contact with Audi.

      stuart

    • @David Dixon – I feel your pain, brother. I had a BMW diesel which claimed to get about 58mpg but I never got any better than about 40. About time the whole system was overhauled to make it more relevant IMHO.

  5. Since 2004, the price of fuel has risen by 240%.

    Reply
  6. Hi, can you put some tips to increase fuel efficiency rather than purchasing new car with higher mileage options.because buying a new car is not good idea, some people don't want to change their loving cars.

    Reply
  7. “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).”

    I’ve been pretty lucky with my previous cars (last 4 were 2002 C5 Diesel Estate, 2001 Mini Cooper, 2004 156 1.9 Diesel, 2002 Volvo V40 1.9D) as they’ve all returned for me the combined figure within a couple of mpg. Maybe a lot of this is living in the Fens, so no hills, and doing nearly all my mileage on single carriageway A roads.

    I have recently bought a brand new Astra 1.0 that has a combined figure of 64mpg. I’m currently seeing just 44mpg on average after a bit over 3000 miles. This is 30% below the combined figure, despite my having the tyres pumped up to the eco pressures, and running without aircon on. I am so disappointed in the economy of this car as I did assume I’d get somewhere near the book figures as I have with all my older cars.

    Reply
    • Stuart Masson

      Hi Steve. Your economy may improve a bit in coming miles, as it can take several thousand miles for the engine to properly run in and achieve its optimal results. However, don’t expect to see a 20mpg improvement…

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