fuel consumption comparison

[Mel]

This is a general question about cars, but apologies if I am in the wrong section.
Question 1: When you look at the official figures for fuel consumption, can you assume that regardless of the make of car, the figures were obtained by a standardised procedure and so are comparable across manufacturers?

Question 2: I am aware that all sorts of factors affect fuel consumption in the real world, driving style, loading, conditions and so on. Is it safe to assume that the same factor will have the same effect on fuel consumption, regardless of the make of car. In other words, let us say that the way that I drive effects fuel consumption by a factor of x, is this x a constant regardless of what car I drive. If so, then fuel consumption figures remain comparable (if not accurate) but if not then fuel consumption figures are not comparable from one make of car to another because environmental factors each have differential effects depending on the manufacturer.

I doubt that there are controlled tests but does anyone have a view?
mel

 

[SirRogerFFS]

One for Prof John I think

 

[Damian]

I think you will find that most, if not all, testing is done on test tracks under controlled conditions, therefore none of the "Official" figures are achievable by the average motorist.
The test conditions do not have the kind of stop/stsrt motoring of the normal driver, with hold ups, traffic lights , roadworks etc

Each persons driving style will change on a daily basis, regardless of whether you think itis exactly the same, you cannot do exactly the same thing at the same time and at the same place every time you drive a particular road, it is impossible, perhaps withthe exception of stopping at junctios and such like permnent fixtures.

 

[JonnyG]

There is to be a new set of EU, nay world standard tests for the so called officail mpg/co figures as the old system is,sorry has always been Naff. different car manufctures have used differing loop holes in the test to err how should i put this "lie" its got so bad that now that they even tape up all the body parts unplug air con hell they even take off the alternator for the EC mpg test.and dont forget its the manufacture who pays for these test and whoever does the test,is kinda on the payrole. worse still stop start technoligy naffs up the figurers even more so,as it works perfectly well given the test cycle but the test cycle itself is nothing what so ever live real road conditions.. having said that the whole cycle is nothing like real road conditions and is so out of date its a joke. even though yes they are so called controlled test.
I suppose for compairing different cars mpg, then yes the test would be a good comparison,that is if you drove the same as the test cycle test..which is impossible.....

 

[philspot1]

According to Honest John (Telegraph), the "cold start" test is done at a temperature of 30 degrees C, so the manufacturers "soak" the vehicle for many hours beforehand so that every part of it has reached the target temp.!! No genuine cold start at all.
Back to question:- yes, I think that your driving style will affect every vehicle you drive in the same way, but not necessarily to the same extent. An obvious difference might be, for me (light right foot, don't believe in hurrying, never try to reach max legal speed for its own sake) driving a light, fuel-efficient car would produce good economy (for that car), while driving a heavy, less efficient car would tend to produce better than average economy).
I don't suppose any everyday driver would or could have the chance to experience a wide range of vehicles on a regular, repeated everyday run. (Test drivers for magazines, etc., are required to check on top speeds, fast cornering, etc, and rarely spend the whole day at 56 mph!)

 

[JonnyG]

funny enough this made news just this week

http://www.bbc.co.uk/news/business-21759258
this is the offical report

http://www.transportenvironment.org/publications/mind-gap-why-official-car-fuel-economy-figures-don’t-match-reality
but strangely this is from a couple of years back
http://www.nextgreencar.com/mpg-monitor/

 

[ProfJohnL]

This is a general question about cars, but apologies if I am in the wrong section.
Question 1: When you look at the official figures for fuel consumption, can you assume that regardless of the make of car, the figures were obtained by a standardised procedure and so are comparable across manufacturers?

Question 2: I am aware that all sorts of factors affect fuel consumption in the real world, driving style, loading, conditions and so on. Is it safe to assume that the same factor will have the same effect on fuel consumption, regardless of the make of car. In other words, let us say that the way that I drive effects fuel consumption by a factor of x, is this x a constant regardless of what car I drive. If so, then fuel consumption figures remain comparable (if not accurate) but if not then fuel consumption figures are not comparable from one make of car to another because environmental factors each have differential effects depending on the manufacturer.

I doubt that there are controlled tests but does anyone have a view?
mel

Let me give uncharacteristically short answers:-
Q1 No
Q2 No

Slightly longer answers:-
Q1 In principal the same test should be applied by each car maker so to some extent the figures should allow you to make general comparisons between the fuel efficecncy of each vehicle. I cant remember where I have seen it but some web site has been gathering real world MPG's for cars, and as a rough guide most only seem to achieve at best about 80% of the claimed MPG's.
Q2 The reasons why the answer to this is no are quite varied, and certainly include those elements already listed in other posts But there are many interactions which affect the overall efficency of an engine, and those efficenecies may change depending on the speed and duty of the engine. So it is quite possible ( if you have the time and the inclination) to compare two makes of car that both return 50Mpg offical figures, but if you were to reduce the speed to say 28MPH then each car may produce very differnt MPG figures

 

[JTQ]

This link gives an insight to the tests:
http://www.dft.gov.uk/vca/fcb/the-fuel-consumption-testing-scheme.asp
From this and other things I am left doubting that wind drag is corrected for; the test being on a rolling road but I have not yet found a reference to that rolling road being in a suitable wind tunnel to replicate drag.
So whilst vehicles might achieve the same EU consumption the one with the poorer aerodynamics from frontal area and drag factor could be worse in real life, particularly so with a high speed content to the driving.
Also the vehicle mass must come into it where the real life driving involves greater amounts of acceleration then the consumption would be higher from this. And if your normal usage involves a lot of sustained high speed usage then a heavier car would be less penalised by its weight . I find this latter point marked with my Disco3; its high speed use is surprisingly good but its mass cripples it in urban and speed fluctuating "A" &"B" road use.

 

[Surfer]

Many vehilces are tested under lab conditions giving the inflated but not false figures. I seem to recall one manufacturer stated that their mpg was based on actual driving conditions, but unsure which one. Either you can blame the EU and government for forcing manufacturers to fiddle the figures. After all if a car was in a higher tax bracket, how many would you sell against the "same" car in a lower tax bracket i.e. a Captiva vs an Antara. Different brands but essentially the same car.

PS I have no idea of the tax bracket of either and they are used as an example.

 

[JonnyG]

Many vehilces are tested under lab conditions giving the inflated but not false figures. I seem to recall one manufacturer stated that their mpg was based on actual driving conditions, but unsure which one. Either you can blame the EU and government for forcing manufacturers to fiddle the figures. After all if a car was in a higher tax bracket, how many would you sell against the "same" car in a lower tax bracket i.e. a Captiva vs an Antara. Different brands but essentially the same car.

PS I have no idea of the tax bracket of either and they are used as an example.

the so called offical mpg figures date back to the 70s as does most of the test procedures.So hardly just one government to blame or indeed the EU! The figures have always been a farce simply because of the test procedures themselves and of course manufacturers have used loopholes to err take the pee.and the taking of the pee pre dates any CO roadtax rating which is something we use,but not ever EU country uses that system for roadtax. As far as i am aware as these test have been the "offical test" in different forms since the 80s, ie city, 56mph, 75 mph mpg test to the modern urban, extra urban and mixed cycle then atleast for the last 3 decades no manufacurer i am aware of would go to the trouble to do another set of mpg figures even though they would be more accurate given "offical " means offical and is the standardised system..

 

[JonnyG]

This link gives an insight to the tests:
http://www.dft.gov.uk/vca/fcb/the-fuel-consumption-testing-scheme.asp
From this and other things I am left doubting that wind drag is corrected for; the test being on a rolling road but I have not yet found a reference to that rolling road being in a suitable wind tunnel to replicate drag.
So whilst vehicles might achieve the same EU consumption the one with the poorer aerodynamics from frontal area and drag factor could be worse in real life, particularly so with a high speed content to the driving.
Also the vehicle mass must come into it where the real life driving involves greater amounts of acceleration then the consumption would be higher from this. And if your normal usage involves a lot of sustained high speed usage then a heavier car would be less penalised by its weight . I find this latter point marked with my Disco3; its high speed use is surprisingly good but its mass cripples it in urban and speed fluctuating "A" &"B" road use.

? the disco3 has a cd of .41 and a cdA of about 1.6 so when you say its high spped use is surprisnly good,exactly what are you comparing it to? its better than say a L/R defender but hardly in the normal modern family car range.

 

[Dustydog]

Q1. I agree no. Well until a worldwide independent body, whoever that may eventually be if at all, does all the testing there are far too many variables . How can tests in USA, Asia, Europe all produce the same environment for testing.

Q2. The best place to find the truth are Owners Forums

 

[JTQ]

? the disco3 has a cd of .41 and a cdA of about 1.6 so when you say its high spped use is surprisnly good,exactly what are you comparing it to? its better than say a L/R defender but hardly in the normal modern family car range.
[/quote]
What I am "exactly" comparing it to is the official fuel consumption figures; actually the question we were asked by Mel to discuss..
The Disco 3, well mine, achieves near identical figures on sustained high speed [but legal] long runs to that quoted by LR for its "Extra Urban" operation.
By comparison my wife's VW Golf mk7, 2.0 TDI has a quoted "Extra Urban" of 78.4 mpg but so far it has not bettered 63 on similar high speed long runs.
The high mass vehicle achieving about 100% of its quoted value, the low mass vehicle achieving 80% of its quoted value.
The Disco3 due to its mass and bulk will never be "good", but it returns what was expected, far different to the Golf which whilst "good" also disappoints. .

 

[JonnyG]

cheers JTQ for explaining that,and yep VW do tend to be able to get some extraordinary claimed figures,thats not to say they arent able to achieve decent mpg..

 

[JonnyG]

Q1. I agree no. Well until a worldwide independent body, whoever that may eventually be if at all, does all the testing there are far too many variables . How can tests in USA, Asia, Europe all produce the same environment for testing.

Q2. The best place to find the truth are Owners Forums

to qoute
""Two test cycles make up the NEDC, the first representing urban or city driving (4.1 miles, 780 seconds), with the second simulating motorway conditions (7.0 miles, 400 seconds). To make the tests repeatable, cycles are conducted on a dynamometer which is set up to represent a flat road, the rollers providing resistance to mimic aerodynamic drag (a fan blows over the car to provide airflow for air intakes). Lookup tables are applied to account for different body shapes, allowing models sharing the same chassis to be tested at the same time.

So you see it doesnt matter where the test are done they are supposed to be standardised.
As for forums being the best place for getting real MPG result from that you can relie on, have to strongly disagree with that,
on lots and lots of counts.firstly no two people will drive the same and no two conditons will be the same, then there are those who swear blind they can work out their fuel mpg based on putting a tenner in,or the trip computers says so and they know for sure its accurate. you know me not one to debate things ha, but i get more laughs from threads on MPG claims than anything else...well almost anything else..

 

[Dustydog]

Q1. I agree no. Well until a worldwide independent body, whoever that may eventually be if at all, does all the testing there are far too many variables . How can tests in USA, Asia, Europe all produce the same environment for testing.

Q2. The best place to find the truth are Owners Forums

to qoute
""Two test cycles make up the NEDC, the first representing urban or city driving (4.1 miles, 780 seconds), with the second simulating motorway conditions (7.0 miles, 400 seconds). To make the tests repeatable, cycles are conducted on a dynamometer which is set up to represent a flat road, the rollers providing resistance to mimic aerodynamic drag (a fan blows over the car to provide airflow for air intakes). Lookup tables are applied to account for different body shapes, allowing models sharing the same chassis to be tested at the same time.

So you see it doesnt matter where the test are done they are supposed to be standardised.
As for forums being the best place for getting real MPG result from that you can relie on, have to strongly disagree with that,
on lots and lots of counts.firstly no two people will drive the same and no two conditons will be the same, then there are those who swear blind they can work out their fuel mpg based on putting a tenner in,or the trip computers says so and they know for sure its accurate. you know me not one to debate things ha, but i get more laughs from threads on MPG claims than anything else...well almost anything else..

The Kia Sorento Owners Forum may prove you wrong Jonny. Have a look. They did quite a detailed survey a few years ago. Most Sorrie owners are tuggers. I agree driving styles do vary but what was apparent from the many submissions the mpg fell well short of the official figures. Most owners own figures were very close indeed.

 

[JonnyG]

Of course you will find exception to the rules,but without me wanting to come across any worse than i normally do, i still recall threads upon thread on sites and even magazines going back 6 or 7 years concerning owners dissapointment with the Kia's Real mpg figures.
Let me try and remember off the top of my head. 2.5ltre 2.5 tonnes and more 135bhp and about 225 ft/lb of torque and the claimed figures where near 30 mpg urban and over 40mpg extra urban? something like that. not knocking the cars ability but maybe ist me those figures wouldnt add up and i wouldnt expect to get to within 5mpg of them figuresunder normal use and i am restrained [tight git]. and then last year or the year before a newer owner of the 175bhp version claimed he regular got 50mpg from a car with a CO rating i think was around 175 CO/km..note regular, again i believe the newer version are good cars and indeed mpg has been vastly improved too butnot to that extent .
I thing what i am trying to say in nice way is did those who bought it actually believe those figures were achieveable and thats why they were so annoyed by the low mpg figures, i include the roadtest done by this mag in that statement. surely experience should play a role in judgement? and of course my opinion isnt solely aimed at Kia's, i use them as an example of a car known well on here for its virtues.....

 

[Mel]

thank you one and all for the intersting points. The reason I asked was that we are thinking about a new tow car and wnated to know if the data supplied by manufacures could be used as a comparator. Clearly not!

mel

 

[Big S]

Mel,

It would appear that this is a Europe wide issue: A report recently published in the ENDS report states:
EU sustainable transport campaign group Transport and Environment (T&E) has called for fuel efficiency tests on new cars and vans to be updated to reflect real-world conditions and prevent manipulation.
The report, prepared by Dutch research organisation TNO, points out that the gap between official test results and real-world driving performance is growing.
It found that in Germany the gap has grown from an average of 7% in 2001 to 23% in 2011, so that only half the anticipated improvement on fuel economy has been achieved on roads. It says that adds some €2,000 to fuel costs over the vehicle’s life.
These results are supported by similar findings in the Netherlands and Switzerland, it says:
The discrepancies threaten to undermine interest in fuel efficient car designs, it warns.
The group’s findings follow a detailed consultancy report for the European Commission which came to similar conclusions.
The report finds three key causes for the growing gap. New technology such as turning off engines when stationary often performs better in the test than in reality. Tests ignore the energy used by an increasing number of systems such as air conditioning, navigation and heated seats. It also stresses that the NEDC road test and laboratory fuel efficiency tests are open to manipulation.
These manipulation practices are now commonplace, with testing facilities being paid to optimise results, for example by improving aerodynamics and lubrication, it alleges. T&E also notes that fuel efficiency data used in several car manufacturer adverts have been successfully challenged.
The report calls for more loopholes to be closed, more stringent monitoring and retesting of current models in production.

 

[Big S]

http://www.transportenvironment.org/publications/mind-gap-why-official-car-fuel-economy-figures-don%E2%80%99t-match-reality
This is the link for the report I mentioned earlier.

 

[Mel]

Thanks Big S. Fascinating. "not fit for purpose" eh. Just goes to show where the more powerful lobby is. If a 1kg bag of sugar did not weigh 1kg, trading standards would be all over it, but you apparantly quote what you like for fuel consumption and CO2 emmissions.
mel