Cars' mpg figures may not be all they're cracked up to be

4 April 2013

Can you imagine a world where product manufacturers might lie stretch the truth present their product in the best possible light in order to persuade you to buy it? That day is here folks. New research from WhatCar? has found that there is a significant difference between official fuel economy information and vehicles' actual performance.


The figures showed that 95.5% of cars do not match the published fuel economy figures, with an average miles per gallon shortfall of 17%. City cars and superminis were the worst culprits, with fuel economy shortfalls of 23.3%, and almost 25% respectively. SUVs demonstrated the lowest shortfall in What Car!'s tests, coming in at only 12.9% down- but then again, they were starting from a lower measure of economy in the first place.

The difference, according to the magazine is that they reviewed fuel economy performance of more than 500 new cars on real roads to arrive at the results - in contrast to official research for published mpg figures which is conducted in laboratories. But it wasn’t all bad news- testing also revealed that some vehicles did deliver the expected miles per gallon, and others exceeded it.

The Mazda 3 outperformed the published average miles by gallon by almost 10%, while the Nissan 370Z exceeded it by 6.8%.

Based on the research, WhatCar% developed an online tool to check cars' fuel economy. The tool asks you to select not only the type of journeys you make, but also the levels of congestion and your style of driving to give you a more accurate estimate of your likely fuel efficiency- and how much this differs from the published figures.

"Expecting high fuel economy and getting the opposite can double a household’s fuel expense," said WhatCar# editor-in-chief Chas Hallett."It is vitally important for consumers to buy the right car for their life."

He also highlighted the "misconception" that smaller cars always give better fuel economy.

"If you use a small-engined car for long motorway runs every day, it will not be that economical," he said, adding that a vehicle with a larger engine would be better.

[email protected] also offer ten handy tips for reducing fuel consumption, including such gems as leaving the kids at home (extra weight), not getting lost (wasting fuel driving around) and to drive your car as if it were a pushbike, freewheeling down hills. All excellent suggestions, of course.

The True MPG calculator only has data for new cars currently on the market- aimed at helping those buying a new car in selecting the most fuel efficient choice for their personal circumstances. But that doesn’t mean you can’t calculate the true mpg for your existing car and driving circumstances- simply put ten gallons of fuel in your car, drive as normal, and see how far you get before you run out of fuel. The answer divided by 10 is your mpg. Simples.


  • SysOp
    Seeing as MPG figures are calculated in highly scientific off-road laboratories, is it any wonder why MPG claims vary significantly? A pinch of common sense please.
  • Captain.Cretin
    Except you are not supposed to do that as it can bugger up the catalytic converter. It has been a looong time since I owned a vehicle that I could reach/exceed the official mpg figures (Audi100 Avant 2.2CD from 1982 and Skoda Felicia 1.3 from 1998), even at 50mph on a long, clear motorway run (M5 from end to end and then back again in the middle of the night).
  • Adam M.
    Still running a 'new' 1997 Peugeot 306 1.9 Turbo Diesel. Its pretty much as good today, as the day I bought it from new, for motorway fuel economy. Tick over has never budged. No fancy electronics to go wrong, passes its MOT every year. Cheap as chips to run, both insurance and Diesel. I still like the sharp angled look of it, compared to modern cars. A classic bit of engineering, the Peugeot Diesel Engine.
  • Grammar N.
    Wasn't this (or an almost identical story) posted on here a month or two ago?!
  • Grammar N.
    Yes, similar though a different source/piece of research -
  • Mike O.
    @Grammar Nazi I thought that to.
  • Grammar N.
    It was, in January - I tried to post the link but its awaiting moderation.
  • chewbacca
    @Grammer Fascist They don't like links on here. And "awaiting moderation" actually means that your comment has disapppeared forever.
  • Zeddy
    Whatcar is a pile of self-righteous crap who loves the HUN (VW group). Try Honestjohn website to see what motorists get themselves.
  • Mr M.
    @ Adam Mason As a previous 306 owner I'll agree they made a cracking engine, but even if they didn't have fancy electrics to go wrong, the bog standard electrics were a disaster waiting to happen. I now own a Mazda 3, so I guess I should be pleased that science has proven I'm a winner.
  • Stu_
    as someone who has a Prius as a company car, it doesn't hit anywhere near the 70mpg that Toyota reckon. The real MPG calculator is scarily accurate around 55mpg.
  • Han S.
    @Grammar Paolo Di Canio "I thought that to." - Surely you mean 'too'?
  • Alexis
    Not sure how you can test cars other than in a lab. You'd need identical weather conditions for a start. Manufacturer X would complain the temperature was a degree out on their day. They're a comparative tool for engines and gearing, not a true MPG indicator. The only problem is that manufactures tailor the ECU mapping to excel at the test.
  • Grammar N.
    @ Hans Solo - I think your question was addressed to the wrong person. Yes, he did mean "too".
  • Stu N.
    I think there's shared blame here. Car manufacturers definitely cheat in the economy tests, taping over panel gaps, overinflating tires and doing anything they can to make the car lighter and more aerodynamic. However, most drivers don't drive like they're on an economy run. Supermini drivers (mostly young people, no kids) especially are likely to give the car a bit of a boot. Claimed MPG figures may be unobtainable, but you can't expect to get close if you drive like a right Clarkson all the time. The one upside to car manufacturer's cheating is that the cars produce fewer emissions in testing than in real life - but VED bands don't realise this. Your 0.9 litre petrol may claim 65mpg and 99g/km CO2, giving free road tax, and if in reality you're only getting 40mpg and 130g/km, your road tax is at least still free.

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