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Petrol vs diesel: Which is the most economical fuel?

Petrol pumpsWhen buying a car, the decision of whether to go for a petrol or diesel model will have a long-term impact on your fuel costs.

The petrol vs diesel debate used to go like this:

Diesel cars hold their value better and can do more miles per gallon.

Their fuel-efficiency means that they're particularly suited to large vehicles such as 4x4s and for motorists doing high mileages.

But on the other hand they're more expensive to buy initially and diesel costs more per litre, so for drivers doing a low mileage, petrol would be better.

Have things changed?

But recently various factors have challenged this logic. For example, the gap between diesel and petrol has risen, with diesel currently about 4p per litre more expensive. while many petrol cars have become more fuel-efficient, narrowing the mpg difference between the two types of vehicles.

These changes have been confirmed in a recent report by consumer watchdog Which? It tested popular cars from superminis to estates and 4x4s, looking at identical-spec petrol and diesel versions of the Ford Fiesta, Vauxhall Astra, VW Tiguan, VW Sharan, BMW 5 Series and Peugeot 308 SW. Which? calculated the annual fuel bill for each based on a typical annual mileage of 10,672.

The organisation discovered taking into account the higher initial cost of the diesel car (which is typically £1,000 to £2,000 on a new car) it could take years before the cost was recouped through fuel savings.

Wait 14 years to see the benefits

For example, with the BMW 5 Series, the diesel version costs £40,945 and delivers 40.9 mpg, whilst the petrol model costs £37,300 and does 34 mpg. But according to Which? It would take up to 14 years (or just over eight years for urban driving) until the fuel savings offset the extra money paid upfront to purchase the car.

However, some diesels did perform better in the report, taking around three years to break even with their petrol-powered siblings – these included the VW Sharan and the Peugeot's 308 SW. The Peugeot diesel costs £18,665 and does an impressive 60mpg, whilst the petrol version is £16,895 and manages 38.7mpg – but on the tested mileage, it still takes about three years for the fuel savings to start paying off.

The Which? survey also considered reliability, taking information directly from their 2012 car survey, which found that petrol cars are generally more reliable than diesels – both in the first three years of their life (the typical warranty period) and even more so between four and eight years old.

‘Petrol might make financial sense’

Which? executive director Richard Lloyd says: “Fuel price rises have been hitting household budgets hard, so it's important for consumers to know they're getting value for money when they buy a car.

“Diesel cars are known for their fuel efficiency, but with lower pump prices for petrol and a premium price tag for diesel cars it may make more financial sense for families to go for the petrol version.”

However for drivers who do need a heavier vehicle or do a high mileage it's still possible that diesel is the right choice. And if so, they'll have plenty of vehicles to choose from as this year is set to be the first in which diesel cars will make up more than half the new car market. In the year up to May 2012 they made up 51 per cent of new car sales, with petrol cars at 47.5 per cent and alternative fuel cars accounting for 1.5  per cent.

Before deciding on which fuel type to choose it's important to do work out your individual requirements – look at the mileage you're likely to do in the coming years, and research comparative costs of petrol and diesel models of the cars you're interested in.

And also don't forget to factor in car insurance costs.

Check out our petrol vs diesel infographic.

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Maria McCarthy

Maria McCarthy

Maria McCarthy is a motoring and lifestyle journalist and author of The Girls' Car Handbook and The Girls' Guide to Losing your L Plates published by Simon and Schuster. She's also a regular on BBC Breakfast news, and local and national radio, commenting on motoring matters. Her pet motoring hates are potholes and high fuel prices.

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