Petrol vs diesel: which is best?

Which fuel should I go for? Let our petrol vs diesel guide answer all your questions on emissions, economy, depreciation and more

BuyaCar team
Jan 6, 2022

The battle between petrol and diesel has been going on for decades, and with the introduction of hybrids, electric and even hydrogen-powered cars, the battlefield has somewhat changed in that time. That said, both fuel types still offer their own benefits. 

The mechanical differences between a petrol and diesel engine are complex, and trying to work out which option will cost you less overall is tricky as new diesel cars typically cost more than equivalent petrol models, but offer the prospect of much higher MPG figures and subsequent lower fuel bills - saving those who cover the most miles the most money.

Meanwhile, petrol cars are typically more refined and can be cheaper to maintain. Then there's the perception that diesel cars are much more polluting than petrol models when the reality is that petrol models typically emit more of certain pollutants while diesels emit more of others.

All in all, there are lots of factors to take into account when making the choice between whether to grab a car that needs the green or the black pump, meaning that the decision of whether to go for petrol or diesel has never been so tough. 

Although electric and hybrid cars are beginning to gain traction in more mainstream markets, petrol cars at least are continuing to sell strongly. The story reads slightly differently for diesel cars, however.

'Dieselgate' - the scandal over real-world diesel emissions - rising fuel prices, the London ULEZ and increased awareness of the harmful emissions produced by older diesel models, have all attributed to the share of new diesel car sales dropping dramatically over recent years.

Manufacturers have invested heavily in cleaning up diesel engines and reducing the amount of emissions they produce - with diesels typically emitting notably lower levels of CO2 than equivalent petrol models - but whether their reputation is harmed beyond repair remains to be seen.

Despite all of this doom and gloom, there are still plenty of reasons why a petrol or diesel car could still be the best option for you. We have all the arguments for which is best right here, so read on for more details.

ISSUES WITH DIESEL PARTICULATE FILTERS (DPFs)

Petrol and diesel: pros and cons


Diesel

Cheaper to run for high-mileage drivers
Best for heavier cars and towing
More economical than petrol
More expensive to buy than petrol cars
May be unreliable if only used for short trips
Older diesel cars will be hit with surcharges

Petrol

More reliable for short and slow journeys
Cheapest to buy
Sportier performance
Less fuel-efficient than diesels
Power not as instant
Lacks pulling power for towing

Choosing between petrol and diesel power

In short, diesels are more expensive to buy and tax but offer better fuel economy. They also provide better pulling power - the shove that you feel when you move away. This makes them well-suited to towing and powering heavier vehicles. Petrols are cheaper to buy and tax, but are less fuel-efficient.

Generally, you’ll have to cover around 15,000 miles a year for the fuel savings of a diesel to outweigh its higher purchase cost.

Emissions are less easy to gauge. Diesels generally emit less carbon dioxide (CO2) than petrol models, but most models produce higher levels of other toxic pollutants.

Petrol and diesel running costs

Petrol vs diesel costs

  • Higher-mileage drivers may find that they can make significant fuel savings with diesel.
  • Petrol will remain best for low-mileage drivers because of the higher cost of diesel cars.
  • Buyers can protect themselves from uncertain future values with PCP and finance deals.
  • Surcharges for high emission vehicles entering cities will become more prevalent across Britain, affecting diesels more than petrol cars.

While fuel prices continue to be pretty unpredictable, diesel is almost always more expensive than unleaded, with a typical difference of around 5-10 pence per litre.

If you're wondering how to predict changing prices, then the answer is that you can't. Currency fluctuations, international sanctions and tax changes are just some of the factors that can cause prices to surge or slump.

It means that if there's little difference in the cost of running a petrol or diesel car, then it's not worth basing your decision solely on this small saving. On the other hand, if the price difference is large, then this is worth taking into consideration.

Purchase price and future value

Despite a fall in demand, diesel cars are usually between £500 to £2,000 more expensive than their petrol counterparts.

But because they typically use less fuel and have lost value at a slower rate than petrol cars, that difference can often be cancelled out or even reversed.

The fact that diesels have held their value well is particularly important because this has helped to make finance payments affordable. Repayments for the most popular type of finance, PCP, are based on the value that a car is expected to lose during the agreement.

As diesel sales continue to fall and more cities start to introduce or increase their existing low emission zone charges and restrictions, it's not known how diesel car prices will be affected.

You can protect yourself against an unexpected drop in value by taking out a PCP finance agreement or leasing a new car. You can hand the car back at the end with nothing more to pay, even if the vehicle has lost value at a faster rate than expected.

Diesel tax and surcharges

Running new and used diesel cars has become more expensive and will continue to do so for many drivers, thanks to tax changes and new charges for diesel cars in some low emission zones.

London’s Ultra Low Emission Zone (ULEZ) incurs daily charges of £12.50 for the majority of diesel drivers. Other cities, including Birmingham and Portsmouth, are preparing to follow suit.

Some local authorities are also introducing higher parking charges for diesel owners. You can read about the different proposals in detail, in our guide to diesel taxes and surcharges. The main points are below:

  • If you drive in central London, a diesel car could prove expensive. Most that were on the road before September 2015 don't meet the latest emissions standards and are affected by the Ultra Low Emission Zone (ULEZ) fee of £12.50 per day. More details
  • Cars that don't meet the latest emissions standards (known as Euro 6) are also more likely to be affected by future clean air zone charges in other British cities. Some authorities are also imposing diesel parking surcharges. Find a Euro 6 car, which includes every new model registered since September 2015, and you have a better chance of avoiding future charges More details
  • The first year of car tax for brand new diesel cars is higher than for petrol vehicles, and the difference can be £300 or more (it's included in the purchase price).
  • Lower carbon dioxide (CO2) emissions from diesel cars have led to lower company car tax, but the Chancellor has increased the surcharge that business users pay, reducing the attractiveness of diesel.

Petrol or diesel: performance and economy

Aside from the cost, there's also the engine's performance to consider. Diesel engines often feel more powerful than petrol engines because you don’t have to rev the engine hard to get the best performance. This makes them great for towing and powering big cars like SUVs. Many of the very latest cars are also considerably cleaner than those that went before them.

However, petrol engines tend to be quieter. With more power available the higher they are revved, these engines feel sportier to drive and often produce an agreeable growl from the exhaust.

Petrol vs diesel emissions

Diesel cars may emit less carbon dioxide (CO2) than petrol models, but they pump out higher levels of other pollutants, including nitrogen oxides (NOx) and tiny pieces of soot called particulates. They are blamed for up to 40,000 early deaths in Britain each year.

That's the reason why diesel vehicles are being targeted as part of efforts to reduce air pollution. The latest emissions regulations should have made diesel cars as clean as petrol versions, but several studies have shown that diesels are still considerably dirtier in real-world driving.

The situation is improving - some brand new diesel cars have recorded low levels of emissions, and a new official European test is being introduced later this year, which will ensure that new cars produce low levels of NOx when driving on public roads.

While you wouldn't want to be stuck in a sealed room with a petrol car running, their exhaust gases are cleaner and have fewer harmful pollutants.

Petrol or diesel for low-mileage drivers

If your next car is only going to be used for low-speed journeys - across town, for example - then it’s almost always best to opt for a petrol car. In these cases, a petrol car is likely to be cheaper anyway, but it’s also likely to be more reliable than a diesel car in these circumstances: modern diesels need regular high-speed runs (on motorways or dual carriageways) to keep their systems working correctly.

It’s because diesel engines produce higher levels of harmful emissions than petrol engines and these include particulates: microscopic soot particles that can embed themselves into lungs, where they can cause breathing difficulties.

Issues with diesel particulate filters (DPFs) for low-mileage drivers

To reduce particulate emissions, modern diesel car exhausts are fitted with diesel particulate filters (DPFs), which trap the harmful particles. When these filters are full, the trapped particles need to be burnt off.

The car’s onboard computers recognise when this is needed and change the engine settings so that the exhaust gas is hotter than usual. This is easily done when driving at high speeds but can be impossible during town driving. That’s why the filters in low mileage cars often get clogged, requiring a visit to the dealer and sometimes costly repair work.

As a result, if you only cover short, low-speed trips, it's worth considering whether it's a wiser idea to go for a petrol or electric car rather than choosing a diesel. 

Petrol vs diesel performance on the road

Diesel-powered cars used to be renowned for being slow, rattly and smoky. Not anymore. You might hear a bit more of a clatter when you start a modern diesel engine but it sounds little different to a petrol motor once it’s warmed up and you’re on the move.

Many drivers prefer the way that diesel cars feel to drive. They produce most of their power at low engine revs, which means that they accelerate strongly from almost the instant that you press the accelerator - petrol engines usually need revving to deliver maximum power.

However, there is a new generation of petrol engines entering the market which provide many of the benefits of diesel engines with fewer drawbacks. These engines are turbocharged to provide more power with good fuel economy and are also powerful without having to be revved.

These include Ford’s small 1.0-litre 'EcoBoost' petrol engine that’s fitted to cars including the Fiesta and Focus, Nissan’s 1.3-litre 'DIG-T' petrol engine for the Nissan Qashqai and the 1.5-litre petrol engine used in the Mini Hatchback and some BMW models. You can rev these engines more than a diesel for extra power, which makes them feel sportier.

Fuel economy is closer to the level of diesel engines, and harmful exhaust emissions are also lower, although some tests suggest that these small turbocharged petrol engines release far more harmful nitrogen dioxide gas on the roads than laboratory tests permit.

Petrol vs diesel running costs calculated: Nissan Qashqai

Take a look at the example below, which is based on petrol and diesel prices remaining the same. If you’re covering 10,000 miles a year and want a previous-generation Nissan Qashqai, then a petrol model will cost you less over three years. Drive 15,000 miles a year, though, and the efficiency of the diesel engine makes up for the higher price - just - making this model the cheapest option.

Car

List price

Fuel economy

Value after 3 years

Running costs (10,000 miles / yr)

Running costs (15,000 miles / yr)

Nissan Qashqai Tekna 1.2 DIG-T 115 (petrol)

£24,290

48.7mpg

£10,687

£16,720

£18,277

Nissan Qashqai Tekna 1.5 dCi 110 (diesel)

£25,910

70.6mpg

£10,882

£17,179

£18,253

 
This example is based on the car’s fuel costs and loss of value over three years, according to cap hpi. Other running costs such as road tax and servicing aren’t included, and neither are new car discounts, which can affect the calculations. We’ve used official mpg figures which are good for comparing the fuel efficiency of different cars but are higher than you’ll see in real-world driving.

Should you choose hybrid or electric power instead of petrol or diesel?

Hybrid and electric cars are becoming increasingly affordable, more suited to long journeys and ever-more efficient. If fuel prices remain high, then you could make impressive savings. All bring large benefits for business users, as they have lower carbon dioxide (CO2) emissions, which results in reduced company car tax.

Pure-electric cars are the top eco choice with no exhaust emissions and cheap electricity top-ups, particularly if you plug in at home. The range of cars on a single charge is rapidly improving; the latest Renault Zoe can travel up to 245 miles before the batteries need a charge. Some more expensive electric cars can go even further, such as the Tesla Model 3 which can achieve up to 360 miles when fitted with the Long Range battery pack. Electric cars are ideal for short distances but, due to their slightly higher purchase price, are more likely to offer savings to long-distance commuters, particularly if they can recharge at work.

Plug-in hybrid cars can also bring substantially lower fuel costs, but only for shorter journeys of around 30-40 miles. When fully charged, these vehicles can typically cover that distance on electric power alone. After this, a petrol or diesel engine takes over; the more this is used, the less you'll save, compared with a conventional car. Not everyone can save with these cars, due to their higher purchase price and limited electric range.

Standard hybrid cars can be a little more expensive than a standard model but bring reasonable fuel savings, particularly in stop-start traffic. They can't be plugged in, and so the batteries are recharged by recovering energy that would usually be lost during braking. The engine is also used for recharging when it's efficient to do so. The electricity drives a motor for power at very low speeds and to assist a standard petrol or diesel engine during acceleration. Long journeys at steady speeds don't play to the strengths of a hybrid system, as there's little energy to recover.

Whether an electric car is right for you will depend on the type of driving you do and whether you can take advantage of a reliable charging socket at work or at home. You can get fast wall chargers installed at home by a number of providers to help speed things up should needs be.

 

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