Are diesel cars still cheaper to run than petrol?

I remember clearly the excitement at the Renault garage I worked for as a mechanic back in 1980, as the first Renault 18 diesel was delivered. I also remember the derision around the garage, the first time the engine was started – it sounded more Massey Ferguson than Renault. At that moment we all thought diesels would never catch on.

How wrong we were, and with the government soon reducing the duty on diesel, to encourage take-up of this inherently more efficient power unit than the gas-guzzling petrol engines of the time, sales soon picked up.

Premium upfront price for diesel cars

Carmakers took advantage of growing demand, charging a premium on diesel cars. This was justified because of the inherent efficiency of diesel engines compared to petrol ones, and because diesel engines have to be more robustly designed than petrol (to withstand much higher compression pressures).

They certainly seemed to be more resistant to failure than petrol. The comparatively simple mechanically controlled fuel-injection systems of the time were well developed and reliable, having been derived from the agricultural and haulage vehicles already employing them.

For a long time, diesel was the first choice for company cars and a growing choice for private buyers too.

The times, they are a-changin’

Back then, the premium to buy a diesel car could easily be offset against the lower fuel price and vastly superior efficiency. And there were added bonuses, when the tie between CO2 emissions and car tax meant diesel road tax was also cheaper.

But the fuel price differential was changed when the government equalised fuel duty and the price of diesel rose above that of petrol as a direct consequence. And the introduction of ultra-efficient lean-burn, super- and turbo-charged petrol engines has made another difference. These offer pretty good economy and a power curve much closer to that of a diesel, but without the seemingly ‘traditional’ premium still charged on every diesel by manufacturers.

It now takes some careful calculation to check which option is best. You have to consider exactly how quickly you can recover the premium paid for a diesel in fuel efficiency savings.

Petrol vs diesel costs investigated

For the first time, the market looks set to sell more diesel than petrol cars this year, so we decided to compare the running costs of petrol and diesel cars for the latest Which? magazine. I was genuinely surprised at how skewed things are towards petrol now.

Carmakers still insist on charging a premium for diesel, so with diesel fuel more expensive and huge improvements in both the efficiency and reliability of petrol engines, it turned out that in four of the six cases we considered, petrol was the clear winner.

For example, with a £2,370 premium on the diesel version of the Vauxhall Astra, we worked out that it would take nearly nine years (getting on for 100,000 miles) to recoup this in diesel fuel savings. In some cases we even found it could take 14 years to see savings!

If I were in the market for a new car, I would certainly check out whether buying a diesel would actually pay in the long run. But I know fuel choice doesn’t always hinge on cost. Emissions, refinement, noise and even towing ability can all come into play. So what really matters to you – are you a diesel or petrolhead?

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