/ Motoring

Are diesel cars still cheaper to run than petrol?

If you had asked me a few years ago about whether a car is cheaper to run on petrol or diesel, I’d have said diesel without hesitation. But the scales appear to be tipping in favour of petrol these days…

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?

If you were considering buying a car, which would be your top choice?

Petrol (40%, 211 Votes)

Diesel (29%, 154 Votes)

Petrol hybrid (10%, 51 Votes)

Diesel hybrid (9%, 46 Votes)

None - I don't drive (5%, 27 Votes)

Electric (4%, 20 Votes)

LPG (liquefied petroleum gas) (3%, 15 Votes)

Total Voters: 526

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DaveSuffolk says:
25 August 2012

I find all these comments about fuel economy slightly amusing, because we waste fuel in so many other ways – by making uneccessary journeys to the shops or school run, by not switching off the engine while waiting in a queue, by carrying uneccessary weight in the boot or not keeping the tyres at the correct pressure. Not to mention erratic driving and braking. Why can’t we just accept that cars cost a lot of money and the little you save on fuel is a drop in the ocean? I drive a diesel because I like it, I like the easy mid-range overtaking, the fact that at 30,000 miles it runs better than it did when new, the low revs at cruising speed, and the extra engine weight to help get traction in the snow. It doesn’t smell and inside the cabin it’s not noisy. I think the price difference to petrol is unjustified and unfair, but as long as it doesn’t get too much worse I’ll buy another diesel next time.

I agree, Dave, and I could add a list of other ways of saving fuel and money. My favourite is to combine visits to places rather than do several out-and-back journeys. It does not worry me that my car sits on the drive for one or two days a week.

I hope you are not planning to replace your car soon. At 30,000 miles, a diesel is just about run-in.

Richard says:
27 August 2012

In my limited experience modern diesel engines are less reliable than petrol. However, I suspect it depends upon the type of use they get. Traditionally diesel engines were considered better than petrol for short journeys. Since the development of low lag turbo engines I think this may no longer be the case.

Six years ago I bought a new Skoda Octavia with the 2 litre TDi engine. With under 50,000 miles on the clock I started to get a problem whereby the engine would suddenly lose power, often at an inconvenient, sometimes dangerous, moment when you need the power. Without displaying any warning light the engine management system would go into a ‘get home’ mode and disable the turbo boost. Stopping the engine and restarting would reset the engine management system and normal power would be restored. This gradually became more frequent to the point where it would happen on most journeys.

I was told that the turbo was probably at fault due to carbon deposits preventing the variable vanes from adjusting freely. Apparently carbon can build up if journeys are short and/or are at low speed.
A Skoda franchise garage estimated a replacement turbo would cost in the order of £1,200 fitted. Fortunately I managed to have the turbo reconditioned through a non-franchise garage for a fraction of that and it’s rectified the problem. The garage told me that a reconditioned turbo should last as long as a new turbo. They said that they recondition a lot of these turbos for the local taxi companies and that the problem isn’t confined to the Volkwagen Audi Group – it’s a general problem with the variable vane technology which is designed to reduce the lag associated with traditional turbos. I understand that carbon build up is not a problem with these type of turbos in petrol engines.

Since then I have had to have a new throttle body because the butterfly valve system failed.

My two cars prior to this diesel were petrol – one was a 6 cylinder, the other a 4 cylinder turbo. I had both from new and kept them to over 100,000 miles. Apart from both requiring new batteries (as you might expect), the engines were faultless.

If you do a lot of long distance driving then diesel may be for you. I don’t. My next car will be petrol!

In my opinion I think diesel engines last a lot longer than petrol engines, I had a 2002 diesel Mondeo which had 127,000 miles on the clock but showed no sign of wearing out or loosing power, I reckon its new owner will be able to get another 127,000 out of it.
Question: It has been reported that diesels will be obsolete in 5 years time, is this true?