Petrol vs diesel cars: Should I buy a diesel car?

Choosing between a petrol and a diesel car is an important decision, but in this advice guide, you’ll find all the facts you need to know to decide which is right for you.

Picking between petrol and diesel power doesn’t just affect how much you pay when you refill your car at the pump. It will impact your economy, your performance, the criteria of your MOT, and other real-world running costs like tax.

And looking a bit further ahead, the sale of new cars powered wholly by petrol or diesel will be banned in the UK by 2030, while hybird cars could continue to be sold until 2035. This could affect which car you choose to buy. For more information, read our detailed post explaining what the 2030 petrol- and diesel-car ban means.

Petrol vs diesel – purchase price

Are petrol cars cheaper than diesel? Or vice versa? The truth is, most of the time, a petrol car is indeed cheaper to buy.

For example, as of November 2020, the least expensive petrol-powered BMW 1 Series – the 118i SE – costs £24,965. Compare this with the entry-level diesel, the 116d SE, which costs £26,485. And not only is the petrol option over £1,000 cheaper – it’s faster (0-62mph in 8.5 seconds, vs the diesel’s 10.3 seconds) and more powerful (140hp vs 116hp).

The trade-off is that the entry-level diesel 1 Series is more economical than the range-starting petrol. It has a claimed fuel efficiency of between 60.1 and 62.8mpg, while the petrol’s is between 45.6 and 47.1 mpg.

So why are diesel cars more expensive to buy? A key factor is that diesel engines usually require extra hardware that petrol ones don’t. To reduce emissions and comply with stricter EU laws, devices such as AdBlue-reliant selective catalytic reduction (SCR) systems often have to be fitted. And these cost money!

Petrol vs diesel – emissions

The difference in fuel economy between petrol and diesel engines is quite dramatic. As a rough rule of thumb diesel cars use an average of 15-20% less fuel than their petrol-powered counterparts – most noticeably on long motorway trips. Using less fuel means that diesel engines also produce less carbon dioxide (CO2), a key contributor to the ongoing threat of climate change.

Although this reduction in CO2 emissions is a great advantage that diesel has over petrol, a diesel engine can produce other harmful by-products. Their exhausts can emit more sooty particles than you’d get from an equivalent petrol-powered alternative, as well as more nitrogen oxide (NOx) gases, which contribute to climate change.

To cut down on cars’ effect on climate change, the European Union introduced rigorous Euro 6d-TEMP emissions testing in 2017. To comply with these new EU rules, many diesel engines were fitted with SCR systems. These use AdBlue (a non-toxic mix of water and urea) and squirt it directly into a diesel car’s exhaust system. Doing this breaks the NOx down into harmless nitrogen and oxygen particles.

Petrol vs diesel – running costs

Whenever you pull up at a petrol station, you almost certainly will have noticed that diesel is more expensive per litre than unleaded petrol. At time of publishing, the average cost of petrol in the UK is 113.9p per litre; diesel costs an average of 117.2p per litre.

Luckily, the average fuel economy of diesel engines more than makes up for this, meaning that – with regard to fuel costs, at least – they are cheaper in the long run. For example, over 10,000 miles, the diesel-powered 1 Series 116d SE should cost around £827.06 in fuel; the petrol 118i SE should cost £1118.38.

Keeping a regular supply of AdBlue handy is a running cost that diesel car owners should be aware of. Luckily, the liquid is cheap and easy to get a hold of – a five-litre bottle can be found on most petrol station shelves and should set you back around £5. Plus, the AA reckons that one litre of AdBlue should last for approximately 600 miles.

Something else to consider with running a diesel car is road tax, which is calculated by CO2 emissions. The amount you pay in tax can vary depending on how old your diesel car is. If it was licensed before 31st March 2017, it will prove cheaper to tax than the nearest petrol-powered equivalent, due to the fact that it produces less CO2. 

The same general rule applies for diesel cars registered on or after 1st April 2017, with tax bands divided by emissions. However, if your car doesn’t meet RDE2 (Real Driving Emissions 2) standards, your first year of tax will be more expensive than that of a petrol model. You can ask your car’s manufacturer if a given model meets these standards.

When it comes to MOTs, diesel and petrol cars are held to slightly different standards. New rules state that if your diesel car has a DPF (diesel particulate filter), you will fail your MOT if the tester notices visible smoke coming from your exhaust pipe(s).

Petrol vs diesel – hybrid vehicles

If neither a petrol or diesel model is drawing you in right now, there are other options you can consider, such as getting a hybrid car. These combine a combustion engine (usually a petrol one) with an electric motor to give it more than one source of power. They’re also much more economical than a petrol or diesel car and – because they emit less CO2 than cars powered by fuel only – tend to be cheaper to tax.

There are four different types of hybrids: traditional (sometimes called “full” or “parallel” or “self-charging”) hybrid cars, mild hybrids, plug-in hybrids and range-extenders.

Traditional hybrids are the most common kind, and include the Toyota Corolla and Prius. They can power the car’s wheels in three different ways: with the engine, the electric motor or both.

The electric motor takes over from the combustion engine at lower speeds or during acceleration. The electric motor can also take lead when the car is coasting. Traditional hybrids – as with almost every other type of hybrid – also use their onboard motors to slow the car down and recharge the battery. This is called regenerative braking. 

Because of all this, a traditional hybrid’s fuel economy tends to be greater than a petrol- or diesel-only car. A Prius’s claimed fuel economy, for example, is 67mpg.

Mild hybrids are similar to traditional ones but, in most cases, the electric motor can only assist the engine – it can’t power the car entirely by itself. Many mild hybrids can coast using just their electric motor, however.

Both traditional and mild hybrids can charge their batteries while the car is on the move, either from regenerative braking or using the combustion engine itself. However, plug-in hybrids – as their name suggests – can also be charged much like an electric car, by being plugged into a power source.

Because of the plug-in technology, plug-in hybrids can also prove more expensive than traditional hybrids. For example, while a Hyundai IONIQ hybrid with a 1.6-litre petrol engine and 141hp costs from £23,840 (as of November 2020), a Hyundai IONIQ plug-in with the same engine and power output costs £30,250.

Despite the price and the fact you’ll likely need to recharge them manually, there are some major up-sides to owning a plug-in over a traditional hybrid. For a start, their batteries are much larger, meaning they can provide more electric-only driving and, in the process, be more fuel-efficient. So, while a plug-in IONIQ may be more than £6,000 dearer, it has a claimed economy of over 256mpg. The regular IONIQ hybrid, on the other hand, returns up to 63mpg.

Finally, range-extenders are predominantly electric cars that have a small petrol engine fitted to recharge their batteries. The engine doesn’t power the car.

Petrol vs diesel – electric cars

While petrol, diesel and hybrid cars make use of a combustion engine, electric cars are powered solely by electric motors and batteries. They have a number of advantages over your traditional combustion engine. They’re quieter for a start, not making the same rumble as you’d hear from a petrol- or diesel-powered car.

Electric cars are cheaper to keep and maintain, too. While topping up a 50-litre tank with diesel would set you back – in November 2020 – an average of £58.60, charging a 100kWh Tesla Model S from empty to full at a public charging point would likely cost around half that. Plus, because they create no harmful emissions, electric cars are exempt from road tax. (The only exception is if you spent over £40,000, in which case you’d pay a one-off “premium supplement” of £320.)

Also, while petrol or diesel cars can come with either manual or automatic gearboxes, electric cars don’t have a clutch, meaning that they can be driven by people with automatic-only licenses.

However, with an electric car, you will most likely have to install a charging station in your own driveway, which can prove costly, even with the government grant that covers up to 75% of the installation fee.

Watch what happens when we drive electric cars until their batteries completely run out

There’s also the fact that, while electric charging can prove cheaper than topping up with fuel, it takes up more time. According to Pod Point, using a 7kW fast charger, it would take eight hours to get a Nissan Leaf from 0% to 100% charge. Because of this, longer charges usually happen in a driveway overnight, while ones that happen at public charging points tend to be the electrical equivalent of a shorter ‘splash and dash’.

‘Range anxiety’ is also a common problem with owners of electric cars. This is the fear that your car will run out of power before you reach your destination, potentially leaving you stranded.

Best electric cars revealed

Petrol vs diesel – on the road…

Is driving a diesel car different to driving a petrol one? In short, yes.

Diesel cars have more torque than petrol ones, meaning that their pulling power is greater. This makes diesel cars ideal if you regularly tow a trailer or caravan. It also means that they may be able to overtake other cars faster. Petrol cars, on the other hand, require more gear changes when overtaking.

Diesel engines are generally noisier than petrol ones, though.

What’s the future for diesel cars?

With so many alternatives – such as hybrids and electric cars – becoming increasingly viable, the future for diesel-powered cars is unclear. However, at the moment, lower CO2 emissions and impressive fuel economy make them worth considering if you spend a lot of your time driving on the motorway. Meanwhile, petrol cars tend to be better if you spend more time driving in town.

Plug-in hybrid cars and electric cars are quickly catching up with combustion engine-powered cars. The latter especially has been given a lot of push, as the UK government hopes for the country to become carbon neutral by 2050 by banning the sale of new wholly petrol- and diesel-powered cars by 2030. Electric charging points are becoming more and more common (see our Electric Car Charging Map to find your nearest ones), and will almost certainly prove cheaper for you than filling up a tank with petrol or diesel.

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