The best performers when it comes to real-life v official economy? 2.5 – 3.0-litre diesels

In the light of increasing publicity surrounding the New European Drive Cycle’s unsuitability for producing miles per gallon figures that are achievable in the real world, Emissions Analytics looks at some of the factors to consider when considering a green car purchase.

Using a real-world test cycle, undertaken on real roads and lasting for over two hours, Emissions Analytics has data on over 90 per cent of new cars sold in the UK. This reveals that nearly 98 per cent of the new cars tested fail to achieve their published figures, the average shortfall is 25 per cent and the worst offenders overstate their claims by as much as 40 per cent.

Does size matter?

So, accepting the fact that nearly all new cars are going to be less green than they purport, we looked to see if there was a correlation between mpg and engine size.

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The graph above (blue line = average of official combined mpg; red line = average of true combined mpg, for different engine sizes) shows that official mpg falls with engine size, that is the larger the engine the less mpg you can expect to achieve. However, it also shows that mid-size engines (1.5-2.5 litres) are the best performing rather than the small engines which are typically marketed as being frugal. The gap between official mpg and Emissions Analytics’ data is 22 per cent for 1.0-1.5 litres, but falls to just 5 per cent for 2.5-3.0 litres. Perhaps it is a product of the manufacturers’ marketing strategies that mpg and CO2 are considered less of a motivating factor for purchasers of larger engine vehicles. So if you’re looking to buy a green car, simple downsizing is not the solution.

Petrol or diesel?

If you didn’t mind the noise and the rough ride, diesel used to be the obvious choice for thrifty drivers. It was cheaper to buy and a tank took you further. In recent years manufacturers have made huge improvements to the diesel engine making it cleaner and more powerful. And, as more people have bought them, so the price at the pump has risen, with diesel now more expensive than petrol. While you may be able to travel further on a tank of diesel, it is unlikely to be as far as the manufacturers would have you believe.

An in-depth look at the CO2 data shows that diesel and petrol engines vary in broadly similar proportions (blue line = diesel; red line = petrol; left axis = true combined mpg variance to official combined; bottom axis = engine sizes).

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Small petrol engines are the worst performers with an average surplus of 67 per cent CO2. Diesels in the larger engine range (2.5-2.99 litres) are closest to the truth with an average excess of just one per cent CO2 and this engine range is the nearest match for petrols too. Once again the data is pointing to the upper range of engines for delivering what they promise.

It is early days for hybrid vehicles and Emissions Analytics hasn’t tested enough of the new generation to present a clear picture of their performance, but what is obvious from the real-world data is that when buying a standard internal combustion engine vehicle, whether petrol or diesel, green claims should be treated with caution.

Footnote from Green Car Guide

This research from Emissions Analytics supports the findings from our own road tests. In our experience, 2-litre diesels are the best performers in terms of coming close to their official mpg figures, and downsized petrol engines with lots of ‘green tech’ can be way off their officlal mpg figures – and that includes hybrids.

See our own road tests, with our own real-life economy results