Car Buyers' Guide: Choosing between diesel or petrol

Do the sums before choosing diesel or petrol. Photo / Thinkstock

I would like to know when a diesel vehicle becomes a better choice than a petrol one. My wife is self-employed and travels extensively to visit her clients. She can easily rack up 600km a week in our 2002 2.3l Mazda 6 (owned from new). I'm looking to upgrade this car as it has just ticked over 200,000km. I know that diesels have road user charges and can cost more to service and maintain compared with petrol vehicles, but would I better off making this switch because my wife is travelling so much? David

Based on your wife's drive pattern, diesel would have to become a consideration when the Mazda is replaced. She is travelling about twice the national average of 14,000km, well past the point where diesel starts to have benefits.

It's not as clear-cut as many would believe, however, as there are factors that can influence the long-term savings of one fuel type over another. Whether you buy new or near-new as opposed to an old, high-mileage second-hand vehicle can create a huge difference in overall savings, especially with diesel-powered vehicles.

Not every situation can be covered, so we will answer based on your situation and make an assumption that your next vehicle will be bought out of the showroom.

We will base our workings around a 50-week period (600km a week) and fuel prices set at $2.10 for petrol and $1.50 for diesel. We will also assume an average fuel consumption (taking into account a mixture of stop-start around town and highway driving) of 10l/100km for petrol and 6l/100km for diesel.

Fuel Costs
Petrol: 3000 litres x $2.10 = $6300

Diesel: 1800 litres x $1.50 = $2700 plus road user charges ($53 each 1000km) = $1590, total $4290

Outcome: A saving of just over $2000 in favour of diesel.

Annual registration
Based on a vehicle up to 2.5 litres

Petrol: $431.04

Diesel: $560.89

Outcome: $129 .85 in favour of petrol.

Service costs
Vehicle servicing is normally based around 15,000km/12-month intervals (whichever comes first) for petrol and diesel engines. The standout difference in service costs is the price and amount of the replacement engine oil. Diesel engines can take up to almost double the amount of a petrol engine and require a specific and expensive type of oil, especially for models with particulate diesel filters. Cost of the replacement oil alone can be more than double that of a petrol engine. As the vehicle ages and odometers increase, the differences in service costs are likely to widen. Most franchises should be able to provide a breakdown of basic service costs up to the first 100,000km or five years of ownership.

Vehicle price
We have looked at the retail price difference on a range of similarly spec'd new vehicles up to 2.5 litres with a petrol and diesel option.

Outcome: $3000 in favour of petrol is a conservative figure with some differences being more than $4000.

Summary
In straight-out fuel costs, the diesel wins comfortably but, as you can see, the gap starts to close quickly when other costs are factored in.

Our figures are open to interpretation and should not be considered accurate comparisons for any one particular make and model. It is designed to provide an awareness of some of the cost differences between owning a petrol- or diesel-powered vehicle. The competitive nature of the new car industry should also be taken into account. Discounts on recommended retail prices are common for both fuel variants, as are ex-demonstrator sales. Service plans where prices are fixed for a certain time/distance period are also becoming more common, as are a set number of routine service costs included in the purchase price (you should have had that experience when you purchased the Mazda).

Resale prices can also favour one type of fuel over another also, depending on specific buyer requirements. What this exercise does highlight is where the annual travel difference is less than 20,000km, diesel is not a clear favourite. The smaller vehicle fleet would also tend to favour petrol where fuel consumption figures continue to drop with improvements in engine technology.

The most important point to consider is the main reasons for vehicle ownership. If a large boat or caravan is being towed long distances regularly, diesel definitely has its benefits in fuel savings and driveability. The same goes when travelling long distances regularly with a fully loaded vehicle. However, when buying high-mileage secondhand diesel vehicles, high potential service and maintenance costs need to be considered.

Bottom line is: don't buy on fuel prices alone and do the sums.