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

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.

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).