Working out whether a diesel- or a petrol-powered car will be cheaper to run over three years is a complicated business.
It isn’t just about fuel economy. Depreciation is the biggest cost in car ownership, so you need check how much the car will be worth after three years, which you can do using our depreciation calculator. You should also consider that diesel-powered cars tend to be more expensive to buy and service than their petrol counterparts, although they also attract lower road tax and insurance charges.
Do diesel cars cost more to buy than petrol ones?
Generally, yes. Broadly speaking, the smaller the car you’re considering, the bigger the premium you’ll be charged for an equivalent diesel model.
For example, choose a budget small SUV like the Renault Captur and private buyers will pay £1400 more for an 89bhp diesel than for an 89bhp petrol version in the same trim.
The difference is much smaller for company car users. The diesel Captur’s CO2 emissions of just 95g/km means it sits in a lower tax band. Despite the 3% tax surcharge applied to diesel company cars, the petrol Captur is only £10 a year cheaper.
If you want to compare the monthly tax on diesel and petrol company cars, use our company car tax calculator.
Private buyers won’t always pay a premium for diesel, though. If you want a fast BMW 5 Series, you’ll pay around £700 more for a petrol 535i compared with the diesel 530d – despite the fact that they offer almost identical pace from 0-62mph.
Which costs more at the pump - petrol or diesel?
At the moment, diesel costs about 6.6p more per litre than petrol - or about 30p more per gallon. That means that filling a 50-litre (11-gallon) tank in a diesel car costs £3.30 more than a petrol one. However, the diesel will also go farther on every gallon of fuel.
If you compare a 40mpg petrol car with a 50mpg diesel car, the former will do 440 miles on that 50-litre tank, while the latter will do 550 miles. In other words, you’re spending 5% more at the pump, but travelling 25% farther on every tank.
Judging the difference only on fuel economy and fill-up costs makes diesel look especially attractive – but that’s why you need to consider the differences in purchase price, retained value, servicing costs and even insurance.
Are diesel cars worth more than petrol cars after three years?
Depends on the car you buy. A Ford Focus 1.0T 100 Ecoboost in Edge trim costs £1100 less than a 1.6 TDCi 115 model in the same spec. After three years, the diesel model is worth £1159 more than its petrol counterpart. In other words, you get your extra investment back (and a little more) come resale time.
It’s not the same situation with cars such as the Mini Convertible, though, which can be more sought after in petrol guise. Some 1.6 diesel versions are £1000 more expensive than the 1.6 petrol, but are worth only £100 more after three years. That means you’ll lose £900 more in depreciation if you choose the diesel.
Ultimately, you should always compare the official fuel economy figures – as well as our True MPG data – to see which cars will cost you less over three years. Even if a petrol car is worth more than its diesel equivalent, it might be much cheaper to fill up, which could justify a higher purchase cost or heavier depreciation.
Which are better to drive - petrol or diesel cars?
You should always test drive a car to see if it suits your needs, and that’s even more true when you’re choosing between a petrol-powered car and a diesel.
It’s no longer the case that diesels are always noisier and less refined than petrol engines. Granted, many modern diesels are still clattery at low revs, but some small turbocharged petrol engines can sound just as coarse and produce just as much vibration.
The big difference remains in how diesel and petrol cars deliver their power. Unless it’s turbocharged, you’ll need to let the engine rev higher before you change gear. In a diesel car, the maximum torque (the ‘shove’ you need to pick up speed) is available at lower revs, so you can change up earlier – this tends to make for a more relaxing drive.
For a more exciting drive, it’s difficult to ignore the racier nature of most petrols – especially if they’ve been tuned for higher performance. That said, cars like Audi’s 309bhp SQ5 show that diesel engines can offer near-supercar pace, not to mention an exhaust noise that sounds like a V-engined petrol.
Don’t just look at the performance and economy figures on a piece of paper, though, because once you’ve driven the cars you may find that the option you prefer is the more expensive to run – but that you’re happy to pay for it.
Are diesel cars always more economical than petrol?
A diesel car will almost always use less fuel than an equivalent petrol model. However, our real-world True MPG test shows the differences between them aren’t always as big as the official figures suggest.
The gap between the official economy of the Audi A3 1.4 TSI 122 and 1.6 TDI 105 is 20.0mpg, but our tests show the difference is only 11.6mpg in real-world driving. What’s more, while the 2.0 TDI 150 version of the same car is less efficient than the 1.6 TDI on paper, it achieved an identical True MPG result of 54.1mpg.
If you’re considering an ‘eco’ version of a new car, you should also check whether it delivers its on-paper fuel economy gain.
According to official figures, the BMW 320d Efficient Dynamics returns 7.5mpg more than the standard 320d – but our tests show the gap is only 1.2mpg. The list prices are identical, so private buyers who don’t benefit from company car tax savings might plump for the standard model.
How can I compare real-world diesel and petrol fuel economy?
If you want to find out what you can really expect to achieve on UK roads, visit whatcar.com/truempg to see the full range of cars we’ve tested.
We add new models to our True MPG database every week, so don’t worry if the car you’re interested in isn’t listed yet.