Different Types of MPG Figure (WLTP Vs NEDC)
MPG (Miles Per Gallon) is an age-old measure of how efficient a car is at consuming fuel. The more miles driven per gallon of fuel used, the lower the cost to fuel the vehicle.
All cars are issued with a set of MPG figures which are independently tested and it's this test which provides the figures manufacturers advertise their cars on. The only problem with this has been that what the manufacturer says you'll achieve and what happens in real life have often been two very different things.
The reason behind this is the outdated test, known as the NEDC (New European Driving Cycle) test. Developed in the 1980's and last updated in 1997, the lab-based test was designed to mimic the average usage of a car in Europe. The figures you saw advertised were reached by combining ‘urban’ driving cycles and an ‘extra-urban’ driving cycle. Part of the problem was that the maximum speed of 30mph was held for just 12 seconds. Extra-urban was similarly short - the test was just 6 minutes in length - and the top speed of 70 mph was reached for just a few seconds.
As inventions as rudimentary as air conditioning became commonplace, which is known to utilise power, the test simply no longer reflected the car usage of today. That twinned with the fact that acceleration speeds used in the test were steady at best, the figures quoted were very often much higher than the average joe on the street would achieve.
NEDC was the test all new cars went through right up until 2017, so the MPG figure you have in your head for most of the cars you've ever known will be that of NEDC.
WLTP (Worldwide Harmonised Light Vehicle Test Procedure) is a whole new real-world based test designed to produce more realistic and real-world emission and efficiency figures. Initially due to be launched in 2023 but fast-tracked to 2017 in part due to the VW Emissions Scandal, all new cars registered from 2019 now must be advertised with the new WLTP test figures.
How and why do the figures differ?
The WLTP test cycle is longer than the NEDC cycle in both time and distance - cars are now driven for 30 minutes and 14 miles, in comparison to 20 minutes and less than 7 miles. Being more in line with real-world conditions, the figures produced under a WLTP test are significantly lower than the NEDC test.
Why Does Any Of This Matter?
While the technical details of how the tests are carried out may not be important, this stuff matters to the average car buyer.
- You're considering a used car. You see the MPG figures of 50 MPG. You see there's an offer on a brand new, newer model version of the same car. The car has a newer, more modern engine, but the new car only achieves 44 MPG. How can engine technology have got worse? Answer - it likely hasn't but the new car is tested to WLTP figures where as the used car is quoting NEDC.
- You're comparing 2 nearly new cars at different retailers, same model, same spec, same engine. Both are '19 registered but one is showing as 47 MPG and one is showing at 54 MPG. This is because one retailer has chosen to list the lower but more accurate WLTP figure, and one has shown the higher, but less realistic NEDC figure. In the real world you would achieve the same MPG in either car.
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