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. 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.
The difference is much smaller for company car users. Under the current system, 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.
In some cases, the premium you pay for a diesel can be very high. If you want a fast BMW 5 Series, you’ll pay around £8,700 more for a petrol 530 SE compared with the diesel 530d SE – despite the fact that they offer a very similar pace from 0-62mph.
Which costs more at the pump – petrol or diesel?
At the moment, diesel costs about 2.4p more per litre than petrol - or about 11p more per gallon. That means that filling a 50-litre (11-gallon) tank in a diesel car costs £1.21 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 a small amount 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?
It depends on the car you buy. A Ford Focus 1.0T Ecoboost 100 in Zetec trim costs £1200 less than a 1.5 TDCi 120 model in the same spec. After three years, the diesel model is worth £1050 more than its petrol counterpart. In other words, you almost get you extra investment back when it becomes time to sell.
It’s not the same situation with cars such as the Mini Convertible, though, which can be more sought after in petrol guise. Some diesel versions are £1750 more expensive than the 1.5 petrol, but are worth only £950 more after three years. That means you’ll lose £800 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 petrol 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 such as Audi’s SQ7 show that diesel engines can offer near-supercar pace, not to mention an exhaust noise that sounds like a performance 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 True MPG test shows the differences between them aren’t always as big as the official figures suggest. Of the top 10 best performing cars we've tested under our new True MPG scheme, only four are diesels. In fact, the top performing car, the Suzuki Celerio, is powered by a 1.0-litre petrol.
If you’re considering an ‘eco’ version of a new car, you should also check whether it delivers its on-paper fuel economy gain.
How can I compare real-world diesel and petrol fuel economy?
Should you buy a diesel car? Or maybe a petrol or electrified model would suit you better? Our free What Fuel? tool can help you decide with four easy questions.
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