Official fuel consumption measurement - Mike Humpherson
Does anybody know precisely how the official fuel consumption figures are measured?

I have seen references to official measurements taken on a rolling road, but this would not take into account variations in wind resistance between different vehicles. Is the rolling road only used for CO2 measurements?

On the urban cycle, when there is a sequence of accelerations and stops, how is the acceleration determined? Is it done at full throttle irrespective of the vehicle, or is there some clever method of making any vehicle accelerate at a pre-determined rate, regardless of engine power etc. (I think that would require a computer controlled "foot" on the accelerator pedal, calibrated for the particular vehicle under test.)

Also, does anybody have any comparative figures between the old 56MPH/75MPH/Urban figures and the new Urban/Extra-Urban/Combined methods? I am interested in comparing the (theoretical) economy of newer vehicles against older vehicles, where only the old type of measurements are available.


Mike Humpherson.
Re: Official fuel consumption measurement - mike harvey
Mike,
My understanding is that a driver drives the car on a rolling road, and has to accelerate, change gear etc keeping a 'mark' exactly aligned with a pre drawn 'standard' graph as progress is made. Therefore wind resistance is not considered. You would have to look up the co-efficient of drag for that. even that is misleading as if 2 cars have an identical co-efficient, one may have a larger frontal area and offer a higher resistance through the air. I even heard that manufacturers geared and tuned cars specifically to to show better fuel consumption figures on these tests, at the expense of overall driveability. Chip manufacturers and re-programmers then make good money putting it right. A right rip off all round, I think.
Good luck with you endeavours! The truth is out there somewhere.
Mike Harvey.
Re: Official fuel consumption measurement - Rod Maxwell
The following link contains details of how the standard test is performed:

www.vcacarfueldata.org.uk/e_howtouse2.htm

I can't find any methods of comparing new with old. The problem was that manufacturers used to come up with methods of fixing the old system - hence the new one - which is meant to be more accurate. Unfortunately, the manufacturers have now found methods of fiddling this.

The problem with the test is that the conditions do not reflect what many people find in day to day life, e.g. stop-start traffic jams, freezing cold mornings etc.

If it's any help, most people I know who have tested this find that their urban figure is somewhere around 30% between the measured urban and combined figure and that their long distance figure (e.g. motorways) is around half way between the combined figure and the extra-urban figure. Does that make any sense?
Re: Official fuel consumption measurement - Mike Humpherson
Thanks Rod, but I had seen the information given on that web site already (it is also available as a booklet).

It does not give enough detail to allow you to really understand the test properly. e.g. how the vehicle acceleration is controlled, and how wind resistance and drag are accounted for.

Mike Humpherson.
Re: Official fuel consumption measurement - Neil
The velocity against time diagram listed on the VCA website suggests acceleration is constant and the urban cycle is performed from cold.

Clearly, drag and wind resistance aren't accounted for.
Re: Official fuel consumption measurement - Stuart Bruce
Neil said...

> Clearly, drag and wind resistance aren't accounted for.

This is not rocket science, surely as the test is on a rolling road its not impossible to arrange some sort of resistance on the rollers to replace the wind resistance etc.
Re: Official fuel consumption measurement - Vin
It seems unlikely to me that wind resistance wouldn't be taken into account, as otherwise, we would still be driving cars shaped like bricks; there would be no advantage in reducing a car's coefficient of drag. I know the Cd also affects the level of wind noise, but I can't believe that this has been the sole reason for cars becoming ever more streamlined.

Someone, somewhere must know.
 

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