Ask Honest John Question of the Week: Are fuel economy figures for PHEVs misleading?
Fri, 26 Jul 2024
Dear Honest John,
"Typical economy figures for petrol plug-in hybrids are scandalous. You see a model you are interested in and look at the running cost section and it says 'depending' on the driving journey you can achieve, for example, an MG HS Trophy, 155 miles on a gallon and a full charge, or an Audi Q3 just over 148 miles again on a gallon of petrol and a full charge.
Everybody or most who look into this know that plug-in hybrids have no recharging or virtually any recharging capability until plugged in.
A European environment agency has tested over 600 or 6000 vehicles (for want of a better memory) and has come to the conclusion that 1 gallon of fuel and a full charge battery plug in ONLY returns 30% of the distance range figures published by retail selling sites. How is this allowed to be published on a daily basis for last few years? It's disgraceful.
- RH
Dear RH,
The current Worldwide Harmonised Light Vehicle Test Procedure (WLTP) is the adopted test regime that was introduced in 2017 and is used by all manufacturers in Europe. It was designed to replace the New European Driving Cycle regime that had its origins in the 1980s but had become unfit for purpose.
The WLTP was designed using real driving data to better represent actual driving styles, a greater range of driving situations and longer distances, higher average and maximum speeds and more strict measurement conditions including ambient temperature and vehicle settings.
Vehicles that were measured under both NEDC and WLTP regimes saw a significant drop in their quoted combined fuel consumption figures and an increase in average CO2 emissions.
Despite this, it is crucial to bear in mind that this is still a laboratory test and its primary purpose is to provide accurate and consistent comparison between vehicles.
At best it can offer a guide to what fuel consumption and emissions is achievable in the real world, but it is not designed for this purpose, as it is impossible to design a test that can account for the possible variables. This is why we have Real MPG, in order to give drivers genuine real world MPG figures.
Vehicle condition, tyre pressures, ambient temperatures, road surface, wind speed and vehicle load are just some of the factors that will affect vehicle performance. And this is before you take into account how variations in driving styles will contribute. Acceleration, braking, coasting and steering inputs will all affect vehicle performance in a myriad of small ways.
Then there is the matter of what constitutes an 'average' journey. As an example, the Real MPG figure for the 2001-2008 Honda Jazz 1.2i is 46.8mpg, versus the official NEDC combined fuel economy 51.4mpg, an impressive 91% score.
However, the Real MPG figure is an average based on the submission of Honest John readers, with figures ranging from 36.0mpg to 58.1mpg, the high figure being significantly better than the official NEDC figure.
This illustrates that even for the exact same model of vehicle the figures that drivers achieve will vary, in this example by more than 50%.
This situation is exaggerated for plug-in hybrids, because they have the capability to travel on electric power alone. Although combined figures of 100mpg and more may seem fantastical, for some owners they will be achievable and for others they will not, simply because of the varying nature of how the vehicle is used.
A PHEV owner that never travels more than the maximum EV range of the vehicle in a single journey and charges whenever possible could theoretically cover thousands of miles without ever using a single drop of fuel.
Any vehicle is capable of achieving its combined fuel consumption figure or even exceeding it if it is driven in the right manner on a journey of the correct profile, it is just that very few drivers operate the vehicles in the most economical manner possible on a journey that matches the profile of the official test.
How much better MPG will a hybrid offer compared to petrol?
