Motoring Advice

Petrol or Diesel – which should I choose and why?

Mar 21, 2017

Petrol or Diesel – which should I choose and why?

This really is the question of the hour. You can pretty much time to the minute when Ireland became a diesel-thirsty nation, and it was when the current motor tax and Vehicle Registration Tax system was introduced in 2008. Prior to that. of course, we had used the old by-engine-capacity system, which favoured petrol power simply because most diesel engines at the time had larger capacities. The switch to a CO2-based system, though, put diesel to the fore, and more or less overnight we went from being a nation that bought 75 per cent petrol engines, to one buying 75 per cent diesel.

Now though, that’s starting to fall back. The increasing sophistication and lower emissions of modern petrol engines, added to the bad publicity swirling around diesel and diesel pollution, is causing Irish buyers to take a step back and really start to think about what’s the right engine for them. At a time when you can buy a seven-seat SUV with a 1.2 turbo petrol engine that has Band A emissions and tax rating, it’s no wonder people are confused.

The first and most important question is mileage. The simple rule of thumb is that if you’re not doing mega-miles, then you are foolish to buy a diesel. You really need to be doing at least 20,000km a year to be able to make back, in savings on fuel, the price difference between a petrol and a diesel car. Let’s put it another way — while it varies from model to model, the price gap between a new petrol car and a new diesel car is roughly in the order of €1,500 to €2,500. That means that, for the price of a diesel car, you can have the same model with a slightly thirstier petrol engine and as much as two-and-a-half-grand to spend on fuel. That buys a lot of petrol.

The only way to bridge that gap is to be doing so much mileage every year that the extra frugality of a diesel engine, especially in long-haul, motorway driving, starts to claw back the difference. At a time when most of us have daily commutes of less than 50km, though, that means most of us really shouldn’t be buying a diesel engine.

Technology throws another spanner in the works with diesel particulate filters. These fine sieves, mounted in the exhaust pipe, are there to collect the fine particles of soot and carbon that diesel engines expel, and which are seriously hazardous to human health because they are carcinogenic. The filters gather the soot, stop it getting out to the atmosphere and then burn it off when the filter fills up. To do that, though, it needs heat, heat from the exhaust, heat which can only be generated by driving for a long time at main-road speeds. Those who only do short mileages, mostly around town, aren’t generating that kind of heat, and are eventually going to end up with a clogged exhaust filter. That can, sometimes, be cleared by taking the car for a long run down the motorway, but often it means replacing a damaged filter and that’s a big expense.

Which is not to say all diesel is bad. In fact, if you’re buying a larger car, it’s really the only viable option as petrol engines and hybrids haven’t yet caught up to the point where they can offer reasonable fuel economy in, say, a large executive saloon or a big SUV. 

Smaller cars and modern petrol engines make a great combo though, and for instance Peugeot has just launched the new 5008, a mid-size SUV, which has seven seats and uses a 1.2-litre 130hp petrol engine that provides adequate performance, decent fuel economy and low emissions. It’s one of a growing band of practical family cars, using downsized petrol engines, which can provide diesel-style economy and emissions without the expense or the low-mileage penalty.

Hybrids offer another good route, and are especially well suited to those who do most of their driving in town. The latest version of the Toyota Prius, the new Kia Niro and Hyundai Ioniq all offer excellent low-speed refinement and actually usually use less fuel pottering around town than they do on the motorway. For urbanites, they’re ideal.

That’s without even considering the fact that, in response to fears over urban pollution and public health, the government is likely to start cracking down on diesel in the next few years. The price of the fuel itself is likely to rise, and it’s possible that cities such as Dublin, Cork, and Galway may follow the examples of Paris, Madrid, Stuttgart, and Athens and begin banning older diesel engines from the city centres. The Department of Environment has just put out a consultation paper, inviting submissions from interested parties, on this very subject.

So, don’t go for diesel by default. It still has its uses, and there are many fine diesel engines out there for those that need them. But make sure you’ve sat down and done your sums before you go out to buy.