Petrol or diesel - which one should you buy?

With huge fluctuations in fuel prices, one of the big questions, when buying a car, is whether to opt for petrol or diesel.

There's no right or wrong answer. This decision can't be based solely on which fuel is cheaper at the pump as there are many factors to consider:

• How many kilometres per year the vehicle will travel
• The size and type of vehicle
• The vehicle's fuel economy
• Purchase price
• Depreciation
• Fuel price
• Road User Charge (RUC)
• Registration costs
• Service and maintenance costs
• Convenience
• Noise and performance
• Environmental impact
• Kilometres travelled

The financial benefits of diesel are generally better if you travel high kilometres. For a car travelling low kilometres (15,000km per year or less) petrol may be the better option.

Large cars and 4WDs with high fuel consumption often will benefit from being diesel powered while smaller cars with petrol engines of around 1.6 litre capacity will generally be very fuel efficient.

During the AA ENERGYWISE Rally 2010 we discovered diesel cars usually travel up to 30% further on one tank of fuel. Fuel economy should be considered in conjunction with other factors such as registration costs, Road User Charges (RUC), purchase price, servicing and depreciation.

Diesel cars used to be considerably more expensive than petrol. These days the price difference may not be as extreme, but it's still worth checking.

Depreciation is a major factor when considering which fuel type to use. In particular, petrol models of large 4WD's tend to depreciate more quickly than their diesel equivalents.

Although fuel prices fluctuate, diesel has remained cheaper at the pump. However, RUC and greater registration fees must be factored in to the overall running costs of diesel vehicles.

RUC costs apply to all cars using fuel that isn't taxed at the source such as diesel, and all vehicles over 3.5 tonnes regardless of fuel type.

The annual relicense fee is a fixed cost regardless of distance travelled. Cost can vary depending on vehicle class designation and fuel type.

Diesel vehicles have traditionally been more expensive to service. However, many modern diesels now have similar service intervals to petrol powered vehicles. Service intervals can vary however and depend on the manufacturer's recommendation.

Ensuring the RUC is up to date is a potential inconvenience of operating a diesel vehicle. However, one convenience is that it requires re-fuelling less frequently.

Older diesel engines tend to be very noisy; however modern technology has greatly improved noise and overall performance levels. Modern common rail diesel engines will not tolerate contaminated fuel so refuelling at a busy site is advisable. Refuelling from a farm storage tank is not recommended.

No engines are good for the environment. However, minimising environmental damage should be of concern to all motorists.

• CO2 (Carbon dioxide) is one of the major greenhouse gases that has been linked to discussions on climate change. Generally, diesel engines emit a lower level of CO2 per kilometre travelled so could be considered better for the environment.
• CO (Carbon monoxide) is a lethal gas produced by petrol engines. While CO emissions have been combated with the use of catalytic convertors, they only work effectively once they have reached operating temperature, so CO is still a by-product of petrol engines during the warm-up phase.
• NOx (Nitrous Oxide) combines with low level ozone to form smog. While NOx can be higher in a new diesel than a new petrol engine, petrol engines produce more NOx over time.
• HC (Hydrocarbon) in fuels contains harmful chemicals including benzene. Benzene has been proved to be highly carcinogenic. HC emissions are much higher in petrol engines than diesel.
• SPM (Suspended Particulate Matter) refers to particles suspended in the air, such as soot, emitted from the exhaust of a diesel engine. While modern diesels are fitted with particulate filters, SPM emissions are still high in older diesel engines.

To summarise the environmental impact of petrol vs diesel engines: diesel comes out on top with regards to CO2, CO and HC. Both are around the same when it comes to NOx and diesel is worse for SPM.

Generally speaking, a large 4WD vehicle doing high mileage would be better off diesel powered and a small hatchback, doing a low mileage may be better being petrol powered.

However, there are many factors to consider, and each factor needs to be considered in conjunction with the others. Ultimately, it comes down to the individual motorist's decision as to which fuel best serves their purpose.