I am vaguely familiar with how the urban and extra urban tests are carried out, but does anyone know if the conditions (temperature, windspeed etc) are specified? The fuel consumption of my Mitsubishi Colt 1.5 Di-D is very dependent on the ambient temperature (worse when cold) - it makes me wonder whether the "official figures" are done under optimum conditions - warm day, no wind, well loosened engine etc
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The test is done indoors on a machine, probably why it's so inaccurate.
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The official tests are done in a lab using a rolling road.
The tests are corrected for different ambient conditions, namely temperature, pressure and humidity.
Windspeed can't be included, because on the road sometimes it is with you, and sometimes against - but there is no wind in the lab, so it isn't an issue.
They are useful for comparing car A with car B.
They are not useful for asking "should my car do 34.987654321 mpg?"
i.e., official figures proivide a standard benchmark, using which one can make a choice between different models, they do not give a guaranteed mpg figure which you will achieve with your car on the road.
Number_Cruncher
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Wind cannot be taken into account since it would average 0 mph as NC says, but what about aeorodynamics. Do they use wind tunnels connected to rolling road? If a car is going at 30 mph on a rolling road, do they push air towards the car at 30 mph to get the fuel economy?
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Torque means nothing without RPM
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Number cruncher - if the tests are corrected for temp, press and humidity, do you know what the standard conditions that they are corrected to, are? I get the impression that in some types of cars you can get nearer to the EU combined figure than others. I have never got anywhere near the quoted extra urban figure for the Colt although I have with previous cars - based on tank averages (not the trip comp). The best tank average I have got is 55mpg and I have bettered that in a Vauxhall Combo DTi van which is taller and heavier!!!!
The effect of temperature is very strong. I find that over the same 7 mile journey starting from cold, the mpg on the trip computer varies from 41mpg at an ambient temp of 0C to 52 mpg at around 18C. I guess that that starting a diesel in cold weather takes a lot more from the battery than in warm weather so that alternator loads are greater when it is cold. There are also other "winter" loads on the alternator - lights, heater fan, heated rear window etc - also a greater likelihood of water on the roads to increase drag.
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The fuel consumption of my Mitsubishi Colt 1.5 Di-D is very dependent on the ambient temperature (worse when cold) -
This is understandable. The fuel enrichment mechanism will be operative for longer when it's cold.
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L\'escargot.
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Many diesel cars have hefty electric heaters to assist warm-up when the ambient temp is below a certain level - I don't know if the Colt does - otherwise the engine would take ages to produce warm air from the heater.
if you're only using the car for relatively short journeys then the heaters will be on all the time and they will whack the fuel consumption.
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The official tests do not include a cold start and the lab. temp and vehicle temp. must be between 20 & 30 deg.C.at the start.The vehicle heater must be set in the "off" position.The road load used to carry out the test will have been observed by the same official certifying authority as the one doing the fuel tests.The road load will be recorded on a level road carrying out a series of runs in both directions-the load will be recorded either by "coastdown"-taking a vehicle upto a high speed and letting it freewheel to zero or by "wheel-torque"-using drive shafts fitted with strain-gauges.
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