Petrol versus diesel cars - which should you choose?

By: Ryan Suckling

When the time comes to purchase a car, whether new or used, one of the first things you’ll need to consider is whether you should commit to diesel or petrol, or even forgo the ‘traditional’ options and consider electric.

As with most things, a lot of it comes down to your preferences, lifestyle and, of course, costs – but exactly how do you make the choice?

Over the last decade, diesel has copped some bad publicity for its environmental impact and risk to human health. Who could forget Volkswagen’s ‘Dieselgate’ scandal back in 2015?

Diesel vehicles do produce much higher levels of noxious emissions which are harmful to our health, but these cars are improving, with added features designed to reduce emissions. And despite the global push-back and health impacts, diesel vehicles still made up one third of all new vehicles sold in Australia in 2021.

According to the Australian Bureau of Statistics, of the 20.1 million registered cars in Australia, 26.4 per cent of those were diesel cars as of January 2021. That’s up from 20.9 per cent in 2016. So, around one in four Aussie cars run on diesel.

Today, there are also electric or hybrid vehicles, which are increasing in popularity as cleaner and healthier options.

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Upfront cost and fuel efficiency

When it comes to deciding between a diesel or petrol-powered car, the upfront driveaway costs and potential fuel consumption and savings are key factors in making the final call. Fuel efficiency and environmental performance are also closely related so it’s certainly worth factoring into your decision. You can search for a car’s fuel efficiency and emissions data on the Australian Government’s Green Vehicle Guide.

For many, the higher upfront price of a diesel vehicle is enough to deter them from any benefits or potential long-term savings.

For example, at the time of writing, if you were to purchase a Hyundai Santa Fe Active with the 3.5-litre petrol engine, you’d be looking an estimated driveaway price of $53,800. If you opt for the 2.2-litre diesel engine, the driveaway price starts from $57,650. That’s a difference of roughly $3,850. Of course, prices vary across make and model, but for a ballpark estimate, expect to fork out an extra $2,500 to $5,000 to go diesel.

Hyundai Santa Fe Active

Across some model ranges, diesel models are often also the top-of-the-range variant, so if you want the premium variant, diesel may be your only choice.

There are, however, savings to be had with diesel in other ways. Diesel fuel can be compressed significantly more before it ignites when compared to petrol, meaning diesel engines can run much higher compression engines than petrols.

When the diesel/air ratio does ignite, it expands over a greater range and more energy can therefore be extracted from it. Diesels can therefore attain much greater fuel efficiency – generally within the range of 30 to 40 per cent more efficient.

The cost of diesel fuel does vary. In recent years diesel has been more expensive than unleaded 91. But in 2021 diesel was, on average, 4 cents cheaper than ULP.

Yet with greater fuel efficiency, you can drive for longer on the tank with fewer trips to the petrol station, which could save you money in the long run.

Another fuel cost factor to consider is that currently, the price of diesel does not fluctuate wildly in the same way that petrol does across what is now a fortnightly fuel price cycle across the Perth metropolitan area.

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Scandal and emissions challenges

The reputation of diesel vehicles suffered a big upset in the Volkswagen scandal of 2015, commonly known as ‘Dieselgate’ or the ‘diesel dupe’.

Initially, it was revealed that the car maker had installed a device on about 500,000 of its diesel vehicles sold in the US that enabled those vehicles only to operate their full emissions control equipment during exhaust emissions testing. This allowed Volkswagen to cheat the emissions test and report inaccurate performance results in order to pass vehicle emissions regulations. Under pressure from the Environmental Protection Agency, Volkswagen eventually admitted to the deception. As a result, the historic car maker was also dealt a massive reputational blow.

It was later revealed the emissions cheating software was fitted to about 10.5 million Volkswagen vehicles sold worldwide.

Volkswagen's headquarters and main plant in Wolfsburg, Germany

Diesel was once heralded as a more environmentally friendly option, largely owing to its relatively lower carbon dioxide (CO2) emissions. But while the harmful effects of CO2 emissions are greater in petrol, the immediate risk to human health is greater from diesel.

With the spotlight on its environmental footprint, health impacts and the steady uptake of electric cars, diesel’s grip on the European market has waned considerably. The drop in demand has intensified after several European cities – including Paris, Rome and Oslo – announced a ban on diesel vehicles by 2024.

But just how bad are its effects on the environment and human health?

Diesel vehicles produce a cocktail of emissions that harm our health and the environment. Diesel vehicles produce more NOx (oxides of nitrogen), and particulate matter (PM10 and PM2.5) – which can be absorbed directly into the bloodstream when inhaled – as well as SOx (oxides of sulphur), carbon monoxide, and polycyclic aromatic hydrocarbons, and numerous other harmful emissions. Each of these, when inhaled, can pose dangers to our health, including impacts to the respiratory system.

Among the worst diesel exhaust emitters are older trucks, heavy machinery and certain older diesel cars. However, most diesel vehicles now come fitted with diesel particulate filters (DPFs) that significantly reduce the amount of harmful particulates getting out of the exhaust pipe in the first place.

Be mindful that these need to be able to burn off the collected soot – called DPF regeneration – at regular intervals. Typically, this requires the vehicle to be driven for long enough for the exhaust system to get hot enough to activate the DPF regeneration process. The emissions from this burning off is much less harmful than raw diesel exhaust. Check the handbook for the specific DPF regeneration requirements of the vehicle you’re considering.

There is also AdBlue, a fluid used in some diesel models to target exhaust fumes to create nitrogen and water vapour.

When looking at diesel vehicles, find out what’s been fitted to reduce harmful emissions.

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AdBlue filler neck on a diesel car

Performance and comfort

With diesel vehicles, you can expect more torque to be available lower in the rev range. That’s why trucks and machinery rely on diesel as it’s better equipped to deal with a heavier load and withstand longer distance driving. This characteristic of diesels can also benefit drivers who regularly tow a caravan, boat or other heavy trailers, especially over long distances. Petrol engines, where similar engine sizes are compared, usually produce more peak power at higher revs, but less torque. However, the gap is narrowing with the increasing popularity of small, turbocharged petrol engines.

When it comes to comfort, the diesel is less smooth and generates more noise, but one advantage of diesel is that the fuel itself is much safer to handle than petrol, which ignites far more readily.

Servicing

Generally, diesel engines cost more to service. Among other things, diesels require engine-specific filters, oils and coolants to run efficiently.

That’s why neglecting to service your diesel is a very bad idea. If you’re planning to buy used, make sure you see the vehicle’s full-service history to establish whether it’s been properly looked after.

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Take the Mazda CX-8 Sport which comes in both a 2.5l petrol and 2.2l diesel tank. Over a period of five years’ servicing, the diesel will cost you an estimated extra $145 - that’s an average of $29 more than the petrol version each year.

Mazda CX-8 Sport

Mazda CX-8 Large SUV Sport 2.5l Petrol Auto FWD

Mazda CX-8 Large SUV Sport 2.2l Diesel Auto AWD

In addition to the cost of basic servicing, the brake fluid ($76) and cabin air filter ($109) need to be replaced in both Mazda vehicles every 2 years. The diesel’s engine fuel filter will need to be replaced every 40,000km to clear out fuel contaminants and reduce exhaust emissions, which costs $115.

With electric motors having fewer moving parts compared to Internal Combustion Engines, you’ll also shell out less on servicing an EV.

Summary

The higher upfront price and running costs of diesel may deter some, but the choice greatly depends on your lifestyle and what you want out of the car. If you drive long distances and want your car to cope with harsher driving conditions, perhaps you’re better suited to the diesel.

For those set on getting into a small, zippy model for smoother driving, you might be better placed with a petrol car.

But for ultimate smoothness and to reduce your impact on the environment and our health, you might consider going electric, avoiding the fuel station experience altogether.

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Last updated: December 2021