The official NEDC combined figure is based on just 6.8 miles of driving at an average of 29.1mph in a laboratory – so it’s no wonder that the official NEDC miles per gallon figures don’t match real-life miles per gallon figures.
The NEDC figure is calculated by carrying out a simulated urban driving test and an extra-urban driving test. The combined figure is the average between the urban and the extra-urban test.
The urban cycle test is over 2.5 miles at an average speed of 9mph. The maximum speed reached is 31mph. The test involves accelerating and slowing down several times as well as driving at steady speeds and with the engine idling. There are four repetitions of this urban, or ECE, cycle. Total time is 3 minutes 15 seconds, with four repetitions.
The extra-urban cycle test is carried out straight after the urban cycle test and covers 4.3 miles at an average speed of 39mph (with a maximum of 75mph). This test involves around 50% steady-speed driving and the rest is acceleration, deceleration and idling. Total time is 6 minutes 40 seconds.
The official NEDC combined figure – for CO2 emissions and miles per gallon – is calculated from an average resulting from the urban and extra-urban tests, weighted by the distances covered. The combined figure is therefore based on just 6.8 miles of driving at an average of 29.1mph.
The NEDC test is not carried out on a real road – instead all tests take place in a laboratory, between 20 and 30 degrees, on a rolling road (the car is on rollers and doesn’t actually move). This rolling road is able to simulate wind resistance due to aerodynamic drag and and vehicle mass.
The test is conducted with all ancillary loads turned off, such as air conditioning compressor and fan, lights, heated rear window, etc.
The cars must be run-in and have been driven for at least 1,800 miles before testing.
No pre-warming is allowed and electrical equipment like air conditioning, lights and the radio is switched off.
The fixed speeds, gear shift points and accelerations of the NEDC test offer possibilities for manufacturers to engage in what has been called ‘cycle beating’ to optimise engine emission performance to the corresponding operating points of the test cycle.
The problems with the current NEDC test are widely acknowledged, therefore a new global harmonized driving cycle, the World Light Test Procedure (WLTP), is currently being developed.
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