Petrol vs diesel: which costs less?

There are many factors that can impact your decision when choosing which fuel to power your car, but one key element is running costs.

Traditionally, for those who rack up a lot of miles and make mostly motorway journeys, diesel cars have been the default choice for the additional fuel economy they offer over petrol equivalents. However, after the recent inflation in fuel prices, the cost of diesel has risen significantly above that of petrol; at the time of writing, the RAC states that diesel is 22p more expensive on average per litre.

So, with that in mind, does it still make sense for high-mileage drivers to head to the black pump, or does the green nozzle make more financial sense?

The experiment

To compare the two fuels, we gathered six cars: three petrol-powered models and their diesel twins. We then tested them back to back on a mixed route of town, A-road and motorway driving to collect real-world economy figures.

The cars we chose included two family hatchbacks, two estate cars and two large SUVs. More specifically, we picked a Peugeot 308 1.2 Puretech, a Peugeot 308 1.5 BlueHDi, a BMW 320i Touring, a BMW 320d Touring, a Genesis GV70 2.5T and a Genesis GV70 2.2D. Why the variety? Well, as part of our investigation, we wanted to see how the fuel cost difference impacts different types of car, which vary in engine size, power output and weight.

Our test route started at a petrol station in Virginia Water, Surrey, where we brimmed the tanks and made our way down to Junction 9 of the M3 towards Winchester. Once we arrived at Junction 9, we swapped drivers and the positions of the cars and followed the same route back to the Virginia Water petrol station, where we refilled the tanks and crunched the numbers. The round trip totalled 84 miles.

Ensuring a fair test

To make the experiment as fair as possible, we prepared each pair of cars identically. We checked that their tyres were inflated to the recommended pressures, set their climate control systems to 20deg C and selected the Comfort (or Normal) driving modes.

For each journey, the three pairs of cars were tested in similar conditions, with an outside temperature of 4deg C (and general cold winter weather). Traffic on the route was mostly minimal for the three test runs, although the BMWs faced a brief build-up of stop-start traffic towards the end of the evening rush hour.

At our chosen fuel station, we paid 151.9p per litre for regular unleaded – slightly higher than the prevailing national UK average (according to the RAC) of 148.35p. For regular diesel, we paid 176.9p per litre – again, higher than the UK average of 170.33p per litre. This meant the difference between the cost of petrol and diesel used in our test was slightly higher than the UK average, at 25p (rather than 22p).

Large SUV: Genesis GV70

The petrol and diesel Genesis GV70s have the most disparity of our three pairs. The petrol GV70 has a slightly bigger but considerably more powerful engine than the diesel; with almost 100bhp extra, it could be expected to use more fuel. But the difference was narrower than you might expect.

The petrol-engined GV70 achieved an average of 33.3mpg – slightly better than the car’s official combined figure of 29.7mpg. It used 11.26 litres of regular unleaded and cost £17.10 to fill up after the trip. Its diesel sibling, meanwhile, achieved 43.0mpg, also beating its official combined figure of 40.0mpg. It used 9.10 litres of regular diesel and cost £16.10 to fill up on our return.

Overall, despite its fuel costing 25p per litre more at our filling station, the GV70 diesel still proved cheaper to run on our journey, at 19p per mile, compared with 21p for the petrol.

If you extrapolate that difference over a typical three-year/30,000-mile ownership period, that’s a total fuel saving of £600 if you choose the GV70 diesel. Factor in that version’s lower purchase price and it looks a decidedly more cost-effective proposition.

Estate car: BMW 3 Series Touring

In the case of the BMWs, we expected to see a much closer set of results. The petrol and diesel versions are much more similar in terms of engine size and power outputs, developing 181bhp and 187bhp respectively. The main difference between them is that the diesel features mild hybrid technology, employing a small 48V electric motor that boosts acceleration at low revs and eases the load on the engine, thus improving fuel economy.

The 320i petrol engine, meanwhile, has no such assistance, but it still managed an impressive 42.4mpg, narrowly beating its official combined average figure of 42.2mpg. The 320i consumed 9.18 litres of petrol on our route and cost £13.94 to refill after the trip.

However, the 320d diesel achieved a much better set of results. It returned an impressive 56.6mpg (again, beating its combined official figure), consuming just 7.20 litres of diesel on the 84-mile trip and costing £12.74 to fill up on our return.

That equates to a cost per mile of 14p for the 320d, versus 16p per mile for the 320i. So, after 30,000 miles or three years, the diesel works out £600 cheaper to fuel.

Whereas the diesel GV70 is cheaper than its petrol sibling, though, the 320d is – at the time of writing – £2610 more expensive to buy than the 320i, with both cars in popular M Sport trim. That difference will take rather more than three years to recover in terms of savings on fuel.

Family car: Peugeot 308

We must look to our family car twins for the most surprising results of our test.

Thanks to its efficient turbocharged 1.2-litre three-cylinder engine, the petrol-powered 308 used just 7.63 litres of regular unleaded and achieved a very impressive 54.2mpg (beating its combined official figure) – costing £11.59 to fill up at the end of our round trip.

So, how does that compare with the diesel? Well, despite achieving 62.7mpg (slightly lower than its official combined figure) and consuming just 6.24 litres of diesel (costing £11.04 to fill up), the overall cost per mile for both the petrol and diesel versions came to the exact same 13p per mile.

So, if the difference in fuel prices remains as it is, neither the petrol nor diesel 308 holds the advantage when you visit the pumps. However, the petrol version is £1400 less expensive to buy in the first place, and that could be the deciding purchase factor if you want to keep costs down.

Petrol vs diesel: Crunching the numbers

Final verdict

The current gulf between average prices of petrol and diesel has had a marked effect on car running costs.

Historically, the rule was that high-mileage drivers were better off with diesel cars, thanks to the cost savings brought
by their greater economy. But that’s no longer necessarily the case; in none of our pairs of cars did the cost per mile vary by more than 2p between the petrol and diesel variants.

In many cases, the difference in purchase cost between a petrol car and its diesel equivalent will be much more consequential; putting aside insurance, maintenance and other variables, it could take years to recoup that price gulf in fuel savings alone.

More than ever, then, your choice of petrol or diesel power might depend on which you prefer from a driver’s standpoint.  The BMW 320d, for example, accelerates faster than the 320i (0-62mph takes 7.2 sec  versus 7.6sec), and doesn’t have to be worked so hard for that performance.

One thing’s for sure, though: the days of diesel being the default fuel for cheap high-mileage motoring are over.

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