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BUYING A CAR

Buying a new car? Petrol or Diesel: which is best for you?


Mark Oastler has been a prominent motoring journalist in print and electronic media for more than three decades. His diverse roles have included more than a decade of motor sport TV commentary for both the Seven and Ten networks, editor of Street Machine magazine, founding editor of Australian Muscle Car magazine and freelance contributor to Wheels, Motor, Auto Action, Unique Cars, V8X, AMC and more. He also provides road test reviews for Carsguide.com.au and is a feature writer and TV host for the popular Shannons Club website.


There was a time when diesel engines were usually found only in trucks, buses, tractors, trains and earthmovers. They had good fuel economy, pulling power and long life but were also considered dirty, noisy, smelly and generally undesirable for everyday passenger car use.

These days,  diesel engines have seriously cleaned up their act. They are now highly refined, much quieter and clean enough to meet increasingly tough exhaust emission standards.

As a result they are now offered as an alternative to petrol engines in many makes and models of passenger cars and SUVs, but deciding which engine is best suited to your needs and particular lifestyle requires carefully weighing up the pros and cons of each to make an informed decision.

The differences between petrol and diesel engines

Without getting too technical, an engine that runs on petrol relies on an electrical circuit with spark plugs to ignite its fuel, whereas a modern common rail diesel engine relies on a simpler combustion process that does not need spark plugs. They do have what are called ‘glow plugs’ to assist in cold starting, but in modern diesels these usually only activate in sub-zero temperatures. However, from an ownership point of view, having to refuel these engines at different bowsers is just one of several important differences you need to be aware of.

Price

You’ll generally find that diesel-powered cars command higher prices than petrol-powered ones, both when they’re new and when it comes time to sell, although the used car price difference reduces with age. This price disparity can be significant, even between petrol and diesel variants of the same make and model.

The most pragmatic approach is to think about how long you are likely to own the vehicle and how far you’ll drive it, because if the petrol used during that time will cost significantly less than the additional cost of the diesel version, then the petrol engine could be a better deal overall. They are also generally less complex and cheaper to service than diesels (see Servicing & Maintenance) which could also sway your decision.

Performance

There was a time when there was a clear distinction between petrol and diesel engine performance. Those wanting brisk acceleration and smooth running chose petrol while those wanting better fuel economy and load-lugging ability (at the expense of some smoothness and speed) opted for diesel.

However, the line between petrol and diesel performance is becoming increasingly fuzzy, as each engine type seems to display more of the other’s strengths with each new model release. This is the result of increasing refinement, advanced turbocharging and ‘intelligent’ automatic transmissions designed to extract the best performance and economy from either engine.

Diesels inherently have more torque (pulling power), making them better suited to carrying heavier loads and towing as well as being potentially more suited to larger vehicles. However, the overall difference in performance between petrol and diesel engines is becoming harder to pick from behind the wheel.

Economy

Diesel engines generally have better fuel economy than petrol engines (up to 30 per cent or more) and the longer the journey, the greater those savings will be. By contrast, if you do mostly short trips, in busy city or suburban environments with lots of stop-start traffic, those savings can diminish to the point that a petrol equivalent can match it – and even surpass it - for fuel economy. And regular short distance driving can also play havoc with a modern diesel’s exhaust filtration system (see Servicing & Maintenance).

So, when deciding between petrol or diesel it’s most important to firstly establish what kind of driving you do. Consider the higher price you pay for a diesel vehicle, the higher cost of diesel fuel and determine if the superior fuel economy will deliver a better outcome for you?

Also keep in mind that official fuel consumption figures claimed by car manufacturers for all engines are achieved in ideal laboratory conditions. They might look great in ads and brochures, but are difficult to achieve in ‘real world’ driving, where they can often be 2.0 to 3.0 litres/100km higher and more. They do however, allow you to make comparisons between prospective vehicles you are considering.

Servicing and maintenance

Petrol engines are generally cheaper to service and their scheduled service intervals are often similar to diesels these days so it’s worth comparing the two, at least for the duration of a new car warranty period, to see how their quoted servicing costs measure up.

A diesel engine can provide many years of trouble-free motoring, but regular maintenance is the key to reliability and long life for any engine. Today’s common rail diesels have very fine engineering tolerances that rely on owners sticking rigidly to the manufacturer’s recommended service intervals, using quality diesel-specific oils, filters and coolants. A neglected diesel can quickly develop big problems which can be very expensive to fix. If buying used, make sure there’s a detailed servicing history (this applies to any vehicle you are looking to purchase).

Modern diesels are also equipped with particulate filters, designed to trap harmful soot-like exhaust contaminants. These filters also self-clean once the engine warms up, but if you only drive short distances (like the daily school drop-off or shops) the engine may never get warm enough for this self-cleaning to occur and the filter can become blocked. This can also be very costly to repair or replace. Some vehicles feature a warning light advising you to take a long drive to get the vehicle up to operating temperature over a long distance to burn these particulates off.

Your call

Choosing between a petrol or diesel engine is obviously getting tougher, because even though they remain fundamentally different in design and function, the variations in their performance characteristics are becoming less pronounced. The key financial decisions are purchase price, fuel economy, cost of fuel,  maintenance costs and projected resale value. How you plan to use the vehicle, though, is equally important in choosing which engine type would be best suited to your driving needs and lifestyle. 

The information is intended to be of a general nature only. We do not accept any legal responsibility for any loss incurred as a result of reliance upon it – please make your own enquiries.

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