Petrol vs diesel

Even before you decide on the make, model or trim-level of your next car, you’ll need to decide which fuel is better suited to your needs: petrol or diesel. Your choice will have a direct effect on your car’s running costs and what your car is like to drive. 

The difference between them isn’t as clear-cut as it used to be. Anyone who last drove a diesel five to 10 years ago will probably think of diesels as economical, but also noisy and dirty. However, engineering advances have started to blur the line between petrols and diesels in terms of refinement, driving characteristics and even fuel economy.

Mpg vs purchase price

It’s easy to become focused on fuel consumption, because you pay for fuel more often than anything else, but a car’s mpg is only part of the story when it comes to motoring costs. Diesel cars’ impressive mpg (up to 30% better than petrol equivalents) makes them very tempting; they cost less per mile despite the higher cost of diesel at UK pumps. However, you need to bear in mind that they also cost more to buy in the first place, due to their specifics mechanics and construction.

This higher price means you’ll only really see the benefit of diesel fuel economy once your fuel savings have offset the higher price of buying a diesel car. Generally speaking, you’ll need to do upwards of 13-15,000 miles per year to make a diesel purchase worthwhile.

For example, a new Ford C-Max 1.6 TDCi 115 Zetec does over 18mpg more than its 1.6 petrol sister, but is more than £1600 dearer.. On economy alone, you would have to cover more than 33,500 miles before you broke even and started truly benefitting from the diesel’s better economy. That’s great if you use the car for a long daily commute and regular holidays in France, but if your driving mainly comprises the school run and the weekly supermarket shop, you’re likely to clocking up just 5000 miles a year, in which case it would take you over six years to make a financial benefit from choosing the diesel C-Max. By this time your car’s warranty will have long since expired and it’s likely to require regular repairs, eating up any fuel savings. Generally, petrol is the better choice for low-mileage drivers.

Put simply, the sooner you can recoup the premium paid for a diesel car, the sooner you’ll genuinely experience the better economy.

Other costs to consider

You’ll need to bear in mind that diesels also cost more to service than petrols, but less to tax and insure, so for the most accurate predication of how much either car might cost you to run, you’ll have to factor these costs in. On top of this, you may want to consider how much either car will be worth if you trade it in; on the whole, diesels may cost more to buy, but they also retain more of their value. 

To save you the time of working it all out, you can find What Car?'s Three-Year Cost for every car model on sale by checking the full specs from What Car? via the link at the bottom of every car review on Mumsnet Cars. The Three-Year Cost is the overall cost of running a car over three years and 36,000 miles, including What Car?'s Target Price (the most you should pay to purchase the car), plus insurance, road tax, servicing, fuel and depreciation. 

However, if you do significantly more or less mileage than 12,000 a year, or you plan to keep the car for longer, you’ll need to do sit down with a calculator to work out exactly what petrol or diesel will mean for your finances. When working out the cost of fuel, check the AA website for the latest petrol and diesel prices.

How realistic are mpg figures?

Manufacturers’ official mpg figures are obtained in laboratories, so it’s not surprising that few motorists can match them in the real world. For a more accurate representation of what you’re really likely to achieve, What Car?’s True MPG service provides fuel consumption figures obtained by real drivers on real roads.

A taxing future

If you want your choice to be future-proof, it’s worth being aware of the possibility that diesel will become more expensive if discussions to tax fuel according to its energy content turn into reality: diesel contains around 10% more energy than petrol. Company car users are already charged a tax surcharge for choosing diesels, although this is more or less cancelled out by the lower starting tax based on diesels’ lower CO2 emissions (this surcharge will be removed in 2016, when company car tax rates will simultaneously go up again).

Driving the difference

Diesel engines are more refined than ever but, generally speaking, petrols are still quieter and smoother. There are always exceptions to the rule, with some small super- and turbocharged petrols coarser than you’d think.

The most distinguishing feature of a modern diesel engine is that it develops maximum torque – the ‘shove’ you need to pick up speed – at lower engine revs than a petrol, so there’s less need to change up and down gears to keep the engine on the boil. You’ll need to let a petrol engine reach higher revs before you change up a gear, and you’ll spend more of your time changing gear.


However, manufacturers are closing the gap by producing petrol engines with diesel-like torque, and diesel engines with petrol-like response. At the same time, new-generation petrol engines are becoming smaller, lighter and more efficient, with no loss of performance and with economy that comes close to diesel cars’.

Diesels round town

One important factor to consider when choosing diesel is the increasing use of diesel particulate filters (DPFs). These aim to reduce the amount of diesel pollutants, which are especially harmful in built-up areas. In mixed driving, these filters work fine, but if your lifestyle is mainly town-based, you should think carefully before buying a DPF-equipped car. A diet of low-speed urban motoring will clog DPF filters, and the manufacturer’s suggested regimen for unclogging them – usually a brisk ‘blow-out’ on the motorway – doesn’t always work, leading to expensive repairs. However, a diesel without a DPF won’t do the environment any favours.

Petrol or diesel calculator

Working out whether a diesel- or a petrol-powered car will be cheaper to run over three years can be a complicated business. Fill in the boxes in What Car?'s bespoke 'Petrol or diesel calculator' and it will tell you the most cost-effective option based on your individual circumstances. The calculator will need to check various figures for the two cars you are comparing - find these via the link at the bottom of every car review on Mumsnet Cars; once on the What Car? specs page for the car, you'll need to select a model, then scroll down and click through to the car's running costs. 

Download the petrol vs diesel calculator (pc version)

Download the petrol vs diesel calculator (Mac version)