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Audi Q7 e-tron

Should you buy a diesel, petrol, hybrid or electric car?

Audi Q7 e-tron

Audi Q7 e-tron

If you’re looking to purchase a new vehicle in 2018, do you buy a diesel, petrol, hybrid or electric car? Many people are confused by all the stories in the media, here are the real, independent facts that you need to know.


Firstly, let’s look at diesels. There’s been a lot of bad press about diesels in the news over the last year or so, which has led to a 17% decline in diesel sales in 2017. This media focus on diesels started with Volkswagen’s Dieselgate saga, which brought the issue of diesel NOx emissions to the attention of the public (although people in the industry have been well aware of diesel emissions for years – Green Car Guide was communicating about the issue of emissions and local air quality back in 2009).

In the most recent Budget, the Chancellor announced a Vehicle Excise Duty tax hike on new diesel cars bought from April 2018 for the first year of ownership, apart from models that meet an emissions standard – the Real Driving Emissions Step 2 – that isn’t even in place yet. This Budget announcement, which could add up to £500 on to the first-year VED rate for diesel drivers, may encourage drivers to keep older, potentially dirtier diesel cars. Or scare people into switching from diesel to other fuels or powertrains that may not give them such high levels of fuel economy or such low levels of CO2 emissions in real-world driving. The existing diesel supplement for company car tax will also increase by one per percent.

There’s also the London T-Charge, which came into force in October 2017, which means that older, dirtier cars registered before 2006 – diesels, but also some petrol cars – will have to pay £10 extra to drive into central London. The motivating factor behind the T-Charge, and behind the general rebellion against diesel, is the impact that diesel emissions have on local air quality; diesel emissions of NOx and particulates are bad for local air quality and therefore bad for human health.

However diesel cars generally offer better fuel economy (20% better on average) – and so cheaper fuel bills – for buyers than petrol cars. And diesel cars have lower CO2 emissions – again, 20% lower on average. The UK and Europe have challenging CO2 emission reduction targets, and car manufacturers will receive heavy fines if they exceed the European fleet average CO2 targets – which many are in danger of doing, and the drop in sales of diesel models isn’t helping.

So because of the air quality issues, diesels aren’t recommended for driving that is mainly in urban areas, but if you instead drive lots of miles up and down the UK’s motorway network each year then a diesel is likely to be the most efficient car for you, and therefore one of the cheapest to run. The latest Euro 6 diesels are also much cleaner in terms of emissions that impact on local air quality than older diesels, particularly NOx, and diesel particulate filters (DPFs), fitted since 2011, eliminate 99% of particulates. And perhaps one of the most important points for many drivers, there are some excellent diesel cars out there. Many manufacturers have diesel engines that are powerful, have lots of torque, and are efficient, delivering an excellent driving experience, especially in larger vehicles. Everyone wants an SUV today, but SUVs are heavy and have poor aerodynamics, meaning poor efficiency – but a diesel engine in a large SUV will result in much better economy than a petrol engine.

So our verdict is that diesel-engined cars are not recommended for driving primarily in built-up areas, but the latest efficient diesels are hard to beat for lots of motorway miles, especially in larger cars. But perhaps the most significant issue is that old diesels can be dirty, but the very latest diesels are the cleanest ever – yet the government is taxing the very latest, cleanest diesels with its VED hike; even though the latest Euro 6 cars – including diesels – are classed as low emission for the purposes of the London Ultra Low Emission Zone that is due to come into force in 2019.


Many people are turning from diesel to petrol. There are now many petrol engines that are almost as efficient as diesels in terms of official economy figures. And there are many ‘downsized’ petrol engines, such as small capacity (around 1-litre) three or even two-cylinder engines. These engines in particular have very high NEDC fuel economy figures – over 60mpg in many cases.

Petrol engines will make sense for many people in smaller cars for primarily local journeys. Petrol engines are lighter than diesels, they’re cheaper to manufacture, and they can offer more instant, flexible responses than many diesels. Critically, they’re cleaner than diesels in terms of emissions that impact on local air quality – ie. primarily NOx and particulates. However the inconvenient truth is that petrol engines aren’t as economical as diesel engines in terms of their official mpg figures – and so their CO2 emissions are higher.

But our main concern is that the latest crop of downsized petrol engines have been engineered to perform extremely well on the NEDC test cycle, hence the amazing-sounding official mpg figures. If these engines are driven out of the low-load NEDC cycle – which they always will be in real-life driving – then the real-world fuel economy drops substantially, and in our experience, the discrepancy between the NEDC figure and real-world driving is worse than with diesel engines.


Hybrids combine a petrol (or sometimes diesel) engine with an electric motor and small battery. They don’t need to be plugged in, as energy is captured when braking, which can then be used to supplement the engine with electric power for short periods and when at standstill. The Toyota Prius is the ‘classic’ hybrid, and it is certainly more efficient – and cleaner – than a petrol equivalent. Hybrids are ideal for lots of stop-start city driving. The will offer less benefit on eg. long motorway journeys, although they will still be efficient in most cases.

Plug-in hybrids

If you want more electric running than a hybrid can offer, then you may want to consider a plug-in hybrid. At the time of writing, all plug-in hybrids on sale in the UK are petrol plug-in hybrids, apart from the Volvo V60 D5 Twin Engine and the recently launched Audi Q7 e-tron, both of which are diesel plug-in hybrids. We’re in a situation where the media is demonising diesels, but petrol-engined cars generally don’t match diesels in terms of fuel economy, and many people don’t see that there’s a viable or affordable pure electric car for them – so plug-in hybrid sales have increased over recent years. This sales boom has been primarily driven by the low levels of benefit in kind for company car drivers if they buy a plug-in hybrid, which can save them thousands of pounds per year in tax.

Car manufacturers have therefore brought plug-in cars to market that have official economy figures in the region of 150mpg, emissions of less than 50g/km CO2, and therefore low BIK rates (as well as eligibility for the UK government plug-in car grant, albeit at the current lower rate of £2500). The Mitsubishi Outlander PHEV has done particularly well with sales in this current tax environment. We ran an Outlander PHEV as a long-term test car for six months. We primarily used it on electric power for duties such as the school run and for local dog walks, with the occasional longer distance adventure up a snowy mountain. Using it in this way, it was mainly propelled on electric power, and so it was economical, and an excellent part-electric 4×4 workhorse.

The big problem with plug-in hybrids is that most are not used in this way. Many are bought because of their low BIK rates, and end up being used for high business mileages. If company car drivers don’t pay for their own fuel, then they don’t care about the 30mpg that a petrol plug-in hybrid SUV will average in real-world driving if used for long motorway journeys without recharging. To understand the reason for this we need to go back to the NEDC fuel economy test.

The NEDC test is conducted over only a very short distance – 6.8 miles – when most of the driving can be done on electric power, hence resulting in official economy figures in the region of 150mpg. Real-life driving hardly ever replicates the low-load and short NEDC test cycle, so when the battery charge on a PHEV has been depleted, a two-tonne SUV carrying the weight of two powertrains – an internal combustion engine and a battery and electric motor – as well as a four-wheel drive system and featuring terrible aerodynamics, will not be economical. Plug-in hybrids such as the Hyundai IONIQ are lighter and more aerodynamic, so deliver much better real-world fuel economy. The new WLTP economy test, which replaces the NEDC test, conducted over a longer, and faster, 14.5 mile cycle, will provide more accurate real-world fuel economy figures for all cars, but especially plug-in hybrids. Expect the WLTP economy figure for plug-in hybrids to drop substantially. This will also have an impact on tax rates in due course. And some manufacturers are currently dropping plug-in hybrids from their model ranges because of concerns over the WLTP economy and emissions figures.

So plug-in hybrids can be a great solution, but only if they are primarily driven on electric power, and used for occasional longer journeys. PHEVs are seen as an interim solution until the majority of cars are pure EVs – or possibly hydrogen fuel cell vehicles – with long ranges.

Electric cars

That brings us to pure electric vehicles. Even just a year ago, the real-life driving range of the average pure EV was around 100 miles. Aside from enthusiastic early-adopters, in the mind of the average motorist, this isn’t a sufficient range to guarantee anxiety-free ownership. However we’re now at the stage where the typical EV will provide at least 150 miles of electric range, and even this jump from 100 to 150 miles of real-world range makes an EV much more practical for many drivers. But we’re just about to enter a phase where lots of new EVs, such as the forthcoming Jaguar I-PACE, will have ranges of around 300 miles, which will match what Tesla offers now. This means that increasing numbers of pure electric cars are now becoming competitive with petrol cars in terms of range.

What about price? Final prices have not yet been confirmed for the Jaguar I-PACE, but a starting price of around £70,000 is expected. This can’t really be described as affordable, but battery prices are coming down, so EVs are expected to be on a price parity with petrol cars very soon. In the meantime, EVs have low running costs, they’re financially attractive to company car drivers in particular, they offer a great driving experience, and of course they’re a solution to the issue of local air quality – as well as the problem of CO2 emissions.


A diesel, petrol, hybrid or electric car? So our recommendation is that, if you’ve never driven an electric car, book a test drive as soon as possible; people’s views about electric cars are transformed when they drive them. In our view, everyone should be made to drive a car such as a Tesla Model X – that really would convince most people about how amazing an electric future can be. If you really don’t feel that a pure EV can work for you, then consider a plug-in hybrid. If a PHEV doesn’t work for you – maybe you’re not able to charge one at home or at work – then look at a hybrid. If there are no hybrids that are suitable, there’s always petrol. But if you’re primarily covering long distances on motorways, with little driving in built-up areas, then, as politically incorrect as it may sound, one of the latest, efficient diesels might be the most economical option. However all that is due to change very soon as more longer range, affordable EVs start appearing…

Paul Clarke

Paul Clarke, Editor, Green Car Guide 

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