Are diesel cars more economical?

Are diesel cars really more economical? In the past, the answer to this question would have been a simple yes. Diesel engines burn their fuel by compressing it until the pressure in each cylinder builds to the point where the fuel has no choice but to combust, simply following the laws of physics. That makes for much more efficient combustion than you get with spark-ignition petrol power, and combined with diesel’s traditional lower costs (because it’s a less-refined fuel) that made it the obvious choice for budget-conscious motorists.

The Irish Government’s decision to move all car taxation to a CO2-based system in 2008 pushed diesel’s popularity beyond all previous heights. By then, car makers had developed turbo-diesel engines with direct injection whose performance, economy, and even refinement were starting to rival petrol engines. Combine that with diesel’s naturally lower CO2 emissions, and you had a winning combination. At one point, 75% of all cars sold in Ireland came with diesel power.

All of that was undone by the diesel emissions scandal, when it was revealed that car makers — most notably Volkswagen — had been cheating on their emissions tests, and under-reporting the amount of nitrogen oxides that engines were emitting. Nitrogen oxides — NOX — are dangerous gases, harmful to human health and these revelations caused governments and legislators to clamp down on diesel. Now, diesel power’s Irish market share is under 30% and falling fast.


But our initial question….

But that doesn’t answer our initial question — are diesel cars actually more economical than alternatives?

The answer is a little bit more complex these days, as petrol engines — and especially hybrid engines — have caught up to the point where diesel’s economy dominance has been significantly eroded.

Previously, for car buyers who made regular long journeys up and down the country, and especially on motorways, buying a diesel car was a no-brainer. The best and most economical models — such as the VW Passat BlueMotion — could average better than 55mpg on a long run, and even crack the 60mpg barrier from time to time.

Diesel’s dominance of long journey/high mileage is coming to an end

Now, however, the best hybrid-engined cars can do the same. Toyota’s hybrid-engined Corolla will easily do better than 62mpg on a given journey, and might even nudge past the 70mpg mark if you’re driving gently around town, where its hybrid powertrain can make the most of its battery assistance. Even the taller C-HR crossover, which uses the same hybrid engine as the Corolla, can pretty well reach 60mpg in daily driving.

It’s not just hybrids that are catching up. Even petrol engines can now reach the economy heights once only scaled by diesel. The new Opel Astra, for example, uses a 1.2-litre turbocharged three-cylinder petrol engine which can average as much as 4.8-litres per 100km in real world conditions — that’s hugely impressive, and there are not many current diesel models which could easily match that.

The other hole in diesel’s waterline is its full-life cost. Even if you could eke out a few more kilometers per litre burned, the technology which has to be fitted to a diesel-engined car to keep its emissions under control — AdBlue injection, particulate filters — is expensive, and so buying a new diesel car is almost always more expensive than a petrol-powered equivalent. Given the improvement in petrol economy, it could take you years, even decades of driving to make back the original extra purchase costs in fuel savings.

That is all to avoid the reckoning with both spiking fuel prices and the climate crisis. The simple fact is that, no matter how frugal a diesel car might be, with pump prices above €2 per litre, running one is going to be significantly more expensive than running an electric car. Yes, electric cars are more expensive again to buy, but the disparity in prices is so much that you’ll very quickly make back the extra. Against an increase of around €700 per annum in the cost of running a diesel-engined car, you could potentially cut your total ‘fuel’ costs to as little as €450 per annum by switching to electric.

According to AA Ireland Head of Communications, Paddy Comyn: “Our recent surveys highlight that there appears to be a greater urgency on the part of the car-buying public to move to electric or partially electric vehicles, with this accelerated by ever-rising fuel costs. It’s more than 40 per cent more expensive to fill an average petrol car for the year now, compared to 12 months ago and with a greater range of electric vehicles on sale now, with 57 different fully electric models now on sale from 27 brands, there is now a very healthy choice.”

So, what’s the final answer?

Diesels can be exceptionally economical, if driven with care and attention but the simple fact now is that petrol and hybrid cars have advanced to the point where diesel’s once-unassailable advantage is gone, and when you take into account the total cost of ownership, it’s hard to still justify a diesel engine anymore. That justification is even worse off when you throw the pump price, electric car charging costs, and the climate crisis into your calculations.