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Petrol Vs diesel cars: is diesel still a good choice?

August 26, 2022 by

While electric and hybrid cars may be very much en vogue, petrol and diesel power are still going strong; we set out the pros and cons of these traditional fuels

Conventional petrol cars make up around 45% of all new-car sales, and while diesel is far less popular than it once was, tens of thousands of diesel cars still find new homes each year.

It’s clear then, that despite the looming ban on sales of new petrol and diesel cars, these vehicles tick all the necessary boxes for millions of drivers. If you’re one of those people and you’re weighing up the odds between petrol and diesel power, our analysis will help you choose the right fuel for your next car.

What’s the difference between petrol and diesel cars?

There are a number of key differences between petrol and diesel cars, both in terms of the ownership and driving experience, and with regard to legislation.

Purchase price

A diesel cars is typically more expensive than an equivalent petrol in almost every case – although not always by much.

As an example,  a petrol BMW 520i in M Sport trim has a recommended manufacturer price of £44,980 as of August 2022, whereas the diesel 520d M Sport is £46,080.

These higher purchase prices are down to a number of factors. One of these is that because diesels ignite fuel by compressing it to very high pressure rather than using a spark plug, the engines need to be stronger. Emission-control systems like diesel particulate filters AdBlue also add to the cost.

Running costs

Vehicle running costs encompass a wide variety of factors, so we’ll take these one at a time.

Insurance: on insurance, things are roughly equal. A diesel car may have slightly higher insurance than an equivalent petrol due to potentially more expensive repair costs, but in general the difference isn’t that sharp.

Servicing: again, diesel cars tend to have more systems (such as the emission-control ones mentioned above) than petrol cars, which can add to servicing costs, though often not by much. It’s worth highlighting that older diesel cars to have worse MoT pass rates than petrol cars, potentially meaning more expense down the road.

AdBlue: modern diesel cars use a system called Selective Catalytic Reduction, and in particular a liquid colloquially known as AdBlue, to reduce harmful emissions of nitrogen dioxide (NOx). AdBlue systems squirt tiny amounts of an ammonia-based liquid into the exhaust fumes to reduce NOx emissions, and AdBlue tanks periodically need refilling. An AdBlue tank might be 10 litres in size, and if bought online AdBlue costs around £20 for that amount. Different cars and driving styles see AdBlue consumed at different rates, but it’s reasonable to expect 10 litres of AdBlue to last 3,000 to 6,000 miles, so the cost is relatively minor.

The AdBlue filler will usually be a blue cap

Fuel costs: this is where diesel cars make their advantages felt. The petrol BMW 520i mentioned earlier officially returns 44.8mpg, whereas the diesel 520d manages 57.6mpg, a 28% advantage, and one that means for high-mileage drivers, a diesel car will quickly make up for any additional purchase and running costs it may entail over its petrol counterpart.

As an example, if a litre of petrol is £1.70 and a litre of diesel is £1.82, filling a 60-litre tank will cost a petrol driver £102 and a diesel motorist £109.20. But the 520i driver will cover 582 miles on a tank, whereas the 520d driver will manage 749 miles, with the petrol driver 17.5 pence per mile, and the diesel motorist 14.6 pence per mile, a saving of 2.9 pence per mile. Over 20,000 miles a year for three years, say (a high annual mileage for which a diesel car is well-suited) the diesel driver will have saved £1,740.

To help you save money at the pumps, we’ve created a fuel price checker which finds you the cheapest petrol and diesel prices in your local area.

Emissions and tax

Older diesel engines are relatively polluting, but thanks to AdBlue and diesel particulate filters (DPFs  – which trap soot from exhaust gasses), modern diesels can be considered effectively as clean as their petrol siblings.

Diesel cars emit much less carbon dioxide (CO2) than petrol cars (this was partly why they used to be encouraged by previous road-tax and company-car-tax regimes) than equivalent petrol cars, too. In fact, the current road tax system sees a car’s first-year rate determined by how much CO2 it emits, so while the BMW 520i will cost £230 for its first year, a 520d will be £190.

Company car tax remains determined by CO2 emissions, too, with the diesel 520d attracting  a 30% Benefit-in-Kind rate, and the petrol 520i getting a 33% rate (precise CO2 emissions and therefore tax costs will be affected by alloy wheel sizes and other factors, so do check figures yourself before any potential purchase).

It’s also worth highlighting that emission zones such as London’s ULEZ (schemes are also starting to appear in other cities) are much tougher on diesel cars. Most such schemes allow petrol cars meeting relatively old Euro 4 emission regulations to enter zones without paying, whereas diesels must typically be Euro 6 compliant. This means, in practice, that a petrol car from 2006 is likely to escape any such charge, but a diesel would need to be much newer – from or so 2015 onwards – to not be liable for charges.

Driving and performance 

There are differences between how petrol and diesel cars drive, but they’re not fundamental ones. We’ll take each in turn.

Noise: while diesel cars have become much more refined over the years, they’re still noisier than an equivalent petrol car (sports cars aside). Often described as a ‘clatter’, the sound produced by a diesel car tends to be less pleasing to the ear than that made by a petrol engine.

Revs/engine speed: diesel cars have a lower rev limit (IE how fast the engine can spin) than petrol cars, typically having a maximum rpm (revolutions per minute) of 5,000, compared to the 6,500rpm a petrol car might have.

Power delivery/torque: diesel cars produce less power but more torque than an equivalent petrol engine (think of power as how hard you push a spanner, and torque as how long the spanner is). This means that diesels are well-suited to towing caravans and the like, and also don’t need to be worked as hard to accelerate, particularly at speed. This easy torque and low-rev nature makes diesels excellent motorway cars, and also partly explains why they use less fuel.

Diesel particulate filters: DPFs absorb tiny particles of soot from a diesel car’s exhaust, trapping it in a filter. The trapped soot needs to be burned off (regenerated) periodically, which can only occur when an engine is up to operating temperature, and the car’s engine and road speed is above a certain level. Fail to regenerate a DPF when required and you risk allowing it to be blocked. This means people who predominantly drive in town and don’t often get above 50mph may be best considering a petrol, hybrid or electric car rather than a diesel. See our guide to DPFs for more information.

Pros and cons of petrol cars

Advantages

  • Cheaper to buy
  • No DPF to clog
  • Engines are typically more refined
  • Potentially lower servicing costs and better MoT pass rates
  • Cheaper fuel

Disadvantages

  • Higher fuel economy
  • Higher CO2 emissions
  • Not so well suited to towing

Pros and cons of diesel cars

Advantages

  • Better fuel efficiency
  • Longer range between refuelling stops
  • Good motorway performance
  • Engine doesn’t need to be worked as hard
  • Low CO2 emissions
  • Well suited to towing

Disadvantages

  • More expensive to buy
  • Potentially higher servicing costs
  • Older models likely to fall foul of emission zones
  • Unappealing engine note
  • Potential for clogged DPFs

What happens after 2035?

The UK government recently pushed back a ban on petrol and diesel cars from 2030 to 2035.

After this date, the UK will ban sales of new petrol and diesel cars that are unable to cover a “significant distance” in zero-emission mode. This effectively means only pure electric cars and plug-in hybrids (PHEVs) can be sold from new after 2030. Most plug-in hybrids have petrol engines, although the Mercedes E-Class E300 de is a diesel PHEV. It remains unclear if more diesel PHEVs will be launched over the remainder of the decade.

Also from 2035, only zero-emission vehicles (IE electric cars and hydrogen cars) will be allowed to be sold from new, banning both petrol and diesel cars from new-car showrooms.

All of this applies only to new cars, though: there is no indication that second-hand petrol and diesel cars will face any restrictions relating to their sale. The jury is out with regard to whether any fresh emission control standards will be introduced, though, as while new zones may be brought in, almost all current zones work on a basis of legislating against pre-Euro 4 petrol cars, and pre-Euro 6 diesels.

Should I just go electric instead?

This is such a complex subject that we have a dedicated page to the topic. It is best to weigh up all the pros and cons of switching to an EV or remaining with petrol or diesel power.

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