Ozukus
Well-known member
The current fuel consumption test follows something called the New European Driving Cycle (NEDC) which is in two parts, an ‘urban’ cycle and an ‘extra-urban’ cycle.
Urban cycle
From a cold start the car is driven following a rigidly defined stop start cycle covering a total of around 2.5 miles at an average of 12mph and briefly reaching a maximum of 31mph. The laboratory temperature is maintained at between 20 and 30C.
Extra-urban
Following straight on from the urban cycle, the Extra-urban cycle also mixes acceleration, deceleration, steady speed and idling. The car briefly reaches a maximum speed of 75mph and covers a total of 4.3 miles at an average of 39mph.
Official combined fuel consumption
The test results in three official fuel consumption figures: urban, extra-urban and a ‘combined’ figure which is a weighted average of the other two.
Why aren’t the results accurate?
Many aspects of the test and test procedure contribute towards the growing gap between official figures and real world experience:
> The basic driving cycle was first developed in the 1970s and, though it has evolved, it is short in duration and dominated by periods of idling, low acceleration and low engine load. It doesn’t adequately represent modern-day driving patterns and vehicle performance.
> The test car only briefly reaches motorway speed so ‘highway’ driving is barely represented.
> Air-conditioning, lights and other electrical loads – which increase fuel consumption in the real world - are all switched off for the test.
> Flexibilities and tolerances in the test procedure can be exploited to achieve a lower overall fuel consumption figure – speed that is within the specified corridor for the test but consistently just below the target speed will result in lower fuel consumption than a speed that is just above.
> The test is conducted in a laboratory at an ambient temperature between 20 and 30 C - considerably higher than the average temperature in the UK throughout the year.
> The inertia load applied to the rolling road to simulate real-world vehicle inertia and aerodynamic drag, can only be varied in discreet steps. A small reduction in vehicle weight, such as removing the standard spare wheel, may be enough to get the car just into the next lowest inertia load band and hence experience lower loads in the test than on the road in the real world. ICCT research shows that a car is five times more likely to be just under an inertia limit than just over.
> Car fuel consumption/performance can be optimised for the speeds/conditions of the test.
> Test conditions may give a distorted view of the true benefits of new technologies - stop-start systems for example will show a relatively high benefit in a test in which idling is over-represented. The car is stationary for about 10% of the NEDC.
> The test doesn’t take account of passengers or other loads
Urban cycle
From a cold start the car is driven following a rigidly defined stop start cycle covering a total of around 2.5 miles at an average of 12mph and briefly reaching a maximum of 31mph. The laboratory temperature is maintained at between 20 and 30C.
Extra-urban
Following straight on from the urban cycle, the Extra-urban cycle also mixes acceleration, deceleration, steady speed and idling. The car briefly reaches a maximum speed of 75mph and covers a total of 4.3 miles at an average of 39mph.
Official combined fuel consumption
The test results in three official fuel consumption figures: urban, extra-urban and a ‘combined’ figure which is a weighted average of the other two.
Why aren’t the results accurate?
Many aspects of the test and test procedure contribute towards the growing gap between official figures and real world experience:
> The basic driving cycle was first developed in the 1970s and, though it has evolved, it is short in duration and dominated by periods of idling, low acceleration and low engine load. It doesn’t adequately represent modern-day driving patterns and vehicle performance.
> The test car only briefly reaches motorway speed so ‘highway’ driving is barely represented.
> Air-conditioning, lights and other electrical loads – which increase fuel consumption in the real world - are all switched off for the test.
> Flexibilities and tolerances in the test procedure can be exploited to achieve a lower overall fuel consumption figure – speed that is within the specified corridor for the test but consistently just below the target speed will result in lower fuel consumption than a speed that is just above.
> The test is conducted in a laboratory at an ambient temperature between 20 and 30 C - considerably higher than the average temperature in the UK throughout the year.
> The inertia load applied to the rolling road to simulate real-world vehicle inertia and aerodynamic drag, can only be varied in discreet steps. A small reduction in vehicle weight, such as removing the standard spare wheel, may be enough to get the car just into the next lowest inertia load band and hence experience lower loads in the test than on the road in the real world. ICCT research shows that a car is five times more likely to be just under an inertia limit than just over.
> Car fuel consumption/performance can be optimised for the speeds/conditions of the test.
> Test conditions may give a distorted view of the true benefits of new technologies - stop-start systems for example will show a relatively high benefit in a test in which idling is over-represented. The car is stationary for about 10% of the NEDC.
> The test doesn’t take account of passengers or other loads
