Petrol or diesel

Should you choose petrol or diesel for your next car? Our guide will help you decide

BuyaCar team
Oct 31, 2017

Trying to choose between petrol and diesel can leave you in a daze. There's the prospect of future diesel charges, see-sawing fuel prices and a new car tax scheme that removes some of the advantages of buying a more economical car.

Then there's the effect of diesel emissions on health and confusion over how clean modern cars really are. At the same time, petrol engines are becoming increasingly efficient. The result is that sales of petrol cars are up by 3% on last year while diesel sales have dropped by 14%.

For many people, a modern petrol is their best bet, as they are generally cheaper, cleaner, exempt from diesel charges and usually better-suited to city driving. Hybrid versions are even cleaner, with low CO2 emissions that cut the cost of company car tax.

But there is still plenty going for diesel models. These are usually more efficient, delivering better mpg and lower CO2 emissions. They 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.

Finally, petrol or diesel is no longer the only choice. Electric cars are the cleanest by far because they produce no exhaust emissions and most models are exempt from road tax. You can read more in our electric car guide or scroll down for more help on choosing between a petrol or diesel car.


Diesel: the good and bad

✔ 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 need regular repairs if only used for short, low-speed journeys.
✘ Older diesel cars will be hit with surcharges

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Petrol: the good and bad

✔ More reliable for limited mileages and lower speed driving
✔ Cheapest to buy
✔ Sportier performance

✘ Less fuel-efficient than diesels
✘ Power not as instant
✘ Fuel is no longer much cheaper than diesel

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Diesel car surcharges

London has already introduced the first low emission zone for cars, with its T-Charge, which began operating in October. It's likely to be just the start, with similar schemes being set up around the country, which are set to target older diesel models in particular. You can read about the different proposals in detail, in our guide to diesel taxes and surcharges. The main points are below:

  • If you live or drive in London, then a diesel car could prove expensive (petrol cars are also better for city driving). Most cars on the road before 2006 are affected by the new £10-a-day T-Charge. This is due to be replaced by an Ultra Low Emissions Zone in 2019: most diesel cars from before September 2015 will be hit by the charge. See full details
  • Cars that don't meet the latest emissions standards (known as Euro 6) are more likely to be affected by future parking or emissions surcharges, particularly in other British cities. Find a Euro 6 car, which includes every new model registered since September 2016, and you have a better (but not guaranteed) chance of avoiding future charges
  • Government ministers have hinted that fuel duty or car tax for diesels could be increased in the Budget on November 22 but it's not yet clear how much they could rise, or whether it will happen at all.
  • Some finance agreements will protect you if demand for diesel cars falls so much that their value plummets. If you take out PCP finance or lease your car, then you won't be liable for a sudden drop in your car's value. 


Is a petrol or diesel car cheaper to run?

Petrol cars are typically £500 to £2,000 cheaper to buy than diesel cars when brand new. Both petrol- and diesel-powered cars lose value at roughly the same rate, which means that it is normally cheaper to buy petrol cars on finance and as used models too.

A litre of diesel fuel might be more expensive than a litre of petrol, but the diesel will power a car much further than the petrol because diesel cars are more efficient. In general, that makes every mile in a diesel car cheaper than in an equivalent petrol model. That's particularly true at the moment because diesel is only 0.7p per litre more expensive than petrol, according to recent AA figures. At the beginning of the year, it was 2.5p more expensive.

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 best-selling 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.

Nissan Qashqai running costs: petrol vs diesel


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)






Nissan Qashqai Tekna 1.5 dCi 110 (diesel)






This example is based on the car’s fuel costs and loss of value over three years, according to cap hpi, which specialises in car valuations. Other running costs such as road tax and servicing aren’t included. We’ve also 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.



Petrol vs diesel 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 when 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 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 becoming available, 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-litre petrol engine that’s fitted to cars including the Ford Fiesta, Ford Focus and Ford B-Max, Nissan’s 1.2-litre petrol engine for the Nissan Qashqai and Nissan Juke, and the 1.5-litre petrol engine used in the Mini Hatchback.

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.

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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.

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. To reduce these 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.


Petrol and diesel fuel prices

For years, the price of diesel was consistently higher than petrol, which cancelled out some of the cost advantages of running a more efficient diesel-powered car. Then, as petrol tumbled in price during 2015, the cost of both fuels moved closer together were virtually identical.

At the beginning of this year, the difference increased again. According to the AA, the average litre of diesel was 2.5p more than petrol. Since then the difference has dropped to 0.7p.

If you're wondering how to interpret these fluctuating prices, then the answer is to understand that prices can fluctuate. So if you decide to buy a diesel because it'll be slightly cheaper to run based on current costs, don't be surprised if prices change and it turns out to be slightly more expensive than a diesel model.


Are diesel or petrol cars best at holding their value?

The biggest cost of owning or financing a car is its reduction in value as it gets older - known as depreciation. It used to be the case that diesel cars depreciated more slowly, which meant that they cost less to run - even if their price was higher in the first place.

However, there’s less of a difference today, so petrol and diesel versions of the same family car will lose value at roughly the same rate. Because diesel cars are normally more expensive than petrol cars in the first place, they will cost a little more in depreciation.

That’s not true of all cars, though. Big, heavy SUVs with petrol engines usually use much more fuel than diesel versions, making them extremely unpopular on the second-hand market. If you buy a new version, you can expect its value to plummet. And that’s before you’ve even paid for the first costly fill-up.

The situation may change if demand for diesel continues to fall. Already, Cap hpi, which predicts used car values, has noticed that diesel cars are worth a little less than expected. It says that a three-year-old diesel car lost 10.5% of its value in the first nine months of the year, compared with the expected figure of 8.5%. Petrol car depreciation is at a record low.

You can protect yourself against an unexpected drop in value by taking out a PCP finance agreement or leasing a new car. In these cases, the risk of heavy depreciation is taken by the finance company.     


Petrol vs diesel car tax

Because diesel cars are efficient, they generally use less fuel per mile than equivalent petrol cars, which means that they emit less carbon dioxide (CO2) from the exhaust.

This usually makes them cheaper to tax if you're buying them new because the cost of road tax is based on CO2 emissions for the first year of ownership. There's no benefit in subsequent years, though, because tax is then charged on a flat rate of £140 a year. The only exceptions are electric cars, which are exempt, and vehicles that had a list price of more than £40,000 when new. These are subject to a surcharge for five years.

Company car tax is also based on CO2 emissions, which should make diesel cars cheaper. However, there is also a 3% surcharge on diesels, which has the effect of levelling the field for many models. If the cheapest company car tax is a priority, then a plug-in hybrid car, which can run on battery power alone for several miles, is likely to be the best option, short of a fully electric car.


Diesel vs petrol emissions

Diesel cars may emit less 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 parts 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 vs diesel hybrids

Hybrid cars reduce emissions by recovering energy that's usually lost during braking and using it to recharge a battery.

Electricity from that battery is then used to drive a motor when you accelerate, which reduces the amount of power required from the engine, cutting fuel use.

Petrol hybrids are most common and these include the Toyota Prius, Hyundai Ioniq and Mercedes C300h. There are also a handful of diesel hybrids, including the Audi Q7 e-tron.

The Audi is also a plug-in model, with a bigger battery that can be charged up when you plug the car in. This provides a range of around 15 miles on electric power alone. Several petrol plug-in hybrids are available too.

On shorter journeys, fuel costs will be minimal because power will come mainly (or entirely) from the batteries. On longer journeys, where the batteries run low, then you'll be primarily be using power from the car's engine, so fuel economy will be little different from a non-hybrid car. In this case, then a diesel hybrid will be more efficient. When the battery runs low in petrol plug-in hybrids such as the Toyota Prius+ and Volvo XC90 T8, you're left with the fuel economy of the petrol engine. And in the case of the heavy Volvo, that's not particularly high.



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