MSE News: Government miles-per-gallon car figures 'unreliable'



  • cepheus
    cepheus Posts: 20,053 Forumite
    edited 29 April 2012 at 9:56PM
    Ultrasonic wrote: »
    What always confuses me about the standardised mpg tests is that they don't appear to include the effect of wind resistance, do they?

    Yes they do include aerodynamic drag if that's what you mean. The entire speed v resistance relationship and inertia specific to the vehicle is pre-programmed into the rolling road dynamometer which takes account of this. Factors which affect fuel consumption which are not taken into account in the test however are: very low starting temperatures, road gradients, meteorological wind speed and direction, excessive loads, and ancillary power requirements, particularly air conditioning.
  • Stephen_Leak
    Stephen_Leak Posts: 8,762
    1,000 Posts Combo Breaker
    edited 1 May 2012 at 1:31PM
    You expect any figures from any government to be reliable?

    And they don't quote any mpg figures for these new electric cars. :)
    The acquisition of wealth is no longer the driving force in my life. :)
  • StophUK
    StophUK Forumite Posts: 1 Newbie
    Hi I found this site useful n finding out the real MPG for helping me narrow down my choice of car. Cheap to buy cheap to run & the best MPG


    sorry you have to put the dot in yourself.
  • clangnuts
    clangnuts Forumite Posts: 188
    Part of the Furniture 100 Posts Combo Breaker
    My car never gets empty. Even when the fuel warning light has been on for 50 miles, and I fill up, there's always at least 9 litres in the tank, so the most I ever put into it is 46 litres. All my mpg calculations are based on that, not on the tanks capacity.

    I suspect many other cars are similar.
  • jeferey
    jeferey Forumite Posts: 4,300
    Mortgage-free Glee!
    Brim to brim calculations are the most accurate way to measure mpg.
    clangnuts is right - I drove over 700 miles on a tank once last year, low fuel light came on, did another 50 miles , indicated range dropped to 0, did another 15 miles, finally got some diesel (previous garage was empty!) and I still had 3 litres left in the tank. Wouldn't recommend it though :eek:

    At the end of the day, the tests are just a pointer to what is possible and if people drove sensibly they would probably achieve the quoted figures. However most people don't seem that bothered and keep on accelerating to the next red traffic light only to brake again, leave engine on at level crossings, drive on motorway at 90, etc., etc. - you make your choice, I haven't got money to burn :D
    If at first you don't succeed, try, try, try - oh bu99er that just cheat :D
  • AlexisV
    AlexisV Forumite Posts: 1,890 Forumite
    I always though the MPG figures were good for comparing similar cars, but I now think the whole system needs to be rethought from scratch.

    • because manufacturers can tune their cars for the test, the results can actually be reversed in real life, throwing the whole 'comparison' factor out of the window.
    • the test are more favourable to certain engine types and a 'sweet spot' can be exploited
    • the tests were designed with older engines with less HP in mind and do not take genuine acceleration characteristics into account
  • fivetide
    fivetide Forumite Posts: 3,811
    Tenth Anniversary 1,000 Posts Combo Breaker
    jeferey wrote: »
    Brim to brim calculations are the most accurate way to measure mpg.
    clangnuts is right - I drove over 700 miles on a tank once last year, low fuel light came on, did another 50 miles , indicated range dropped to 0, did another 15 miles, finally got some diesel (previous garage was empty!) and I still had 3 litres left in the tank. Wouldn't recommend it though :eek:

    That reminds me of another TG episode when Clarkson drove an Audi from London to Edinburgh on one tank of diesel. Claimed Audi had said it couldn't be done and the car shut down the computer when it hit zero miles to go but he kept driving and made it back where he had started from.

    What if there was no such thing as a rhetorical question?
  • Ultrasonic
    Ultrasonic Forumite Posts: 4,235
    Part of the Furniture 1,000 Posts Name Dropper Photogenic
    edited 1 May 2012 at 5:18PM
    HappyMJ wrote: »
    I've spent 3 weeks driving a brand new 2012 Kia Picanto 2 Ecodynamics 1.25L car. Supposed to be 65mpg. So far £40 of fuel has been put into the car and has travelled 300 miles with a mix of urban and non-urban and now on empty. Average price paid £1.40 per litre. By my calculation I think that's around 43mpg.

    Guess you know this, but you can't really calculate fuel economy very accurately that way since you'll never know when you've used that £40 worth of fuel all up, or indeed if you've already used more than this. But for what it's worth if you had used £40 of fuel @ £1.40 per litre to drive 300 miles that would be a fuel efficiency of 47.7 mpg (1L = 4.546 UK gallons).

    At the risk of stating the obvious, to calculate fuel economy yourself you really need to use the 'brim to brim' method. That is, fill your car up till the first click on a fuel pump. Reset your trip computer or just note down the odometer reading. The next time you come to fill up do so again to the first click of the fuel pump. The volume of fuel used to fill the car up is then the amount needed to drive the distance you have covered from your last fill up. (Yes there will be variations over exactly when an individual fuel pump clicks off, but this is still your best estimate. If you want to do better, repeat this test several times and combine all of the data.)
  • almillar
    almillar Forumite Posts: 8,621
    Part of the Furniture 1,000 Posts Photogenic Name Dropper
    This is a misleading headline written, I guess, by a journalist looking for attention on a slow news day.
    The figures are 100% accurate - they're tested under laboratory conditions, for the same 2 driving patterns, for all cars.
    Then school run mum comes on saying she sits at traffic lights and start-stops and has used up 1/4 of a tank and the computer only says 20mpg?
    Who do you believe?
    Everyone has different driving patterns, plenty of people don't even have a pattern.
    In your current car, if it's in good condition, and the official figures say a combined 30 mpg, and you get 20, then it's safe to assume that for your driving pattern, you can knock 1/3 off ALL combined figures for ALL cars. Then you won't be so disappointed with your next car.
    I know, for example that I can just about get the combined figure with my driving pattern, and that's what I have done in my last few cars. Longer runs, or 'enthusiastic' runs will affect this of course, but if you're not calculating your MPG properly (brim to brim as above) then don't bother posting about your MPG as it's probably inaccurate.
  • midwinter
    midwinter Forumite Posts: 3,605
    Tenth Anniversary 1,000 Posts Combo Breaker
    Quoted from article on main page:

    The Government's figure for the Ford Focus 1.6TDci 115 Zetec was 67.3 compared with What Car's 43.1.

    I updated my focus this year, for the same model, but newer, and looked at the quoted MPG's before I decided to buy.

    Of course, new model comes with all mod cons, such as an on board computer to work out fuel use etc. Was horrified after filling it, and fuel cost £15 more than I'd usually put in, to see the comupter reckoned I'd do 500 miles before it was empty! My heart sank, was this the worst purchase ever?!

    I'd previously had 58mpg with my old focus, so had expected to get at least that on the new one.

    Over the weeks, the onboard computer is giving results nearer to what I'd expect now. But working out the MPG manually, is consistently giving me values of 63mpg + . I do at least 400 miles per week, major Axx road, some urban, last week was 900 miles, and I do really think a lot of the mileage one gets, is down to driving style. And that doesn't mean slow ! I aim for the least amount of fuel comsumption possible, to keep costs down on the travelling that I can't not do.
    The mind is like a parachute. It doesn't work unless it's open
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