Time to consider diesel?

In times of high fuel prices, it is worth considering a diesel-powered car, Otago Daily Times motoring writer David Thomson says.

If you want to look at what happens when fuel prices really rocket, look at the United Kingdom, he says. In 1990, less than 10% of new cars sold in the UK were diesel. In July 2010, diesel-car sales passed the 50% mark in the UK for the first time.

There's also been a marked increase in diesel car sales in New Zealand; in June 2010, sales of new diesel-powered cars exceeded 20% of total sales for the first time.

"The diesel combustion process, which burns fuel at a much higher temperature and pressure, is more efficient than the petrol combustion process," Mr Thomson said. "Also, diesel fuel has a higher energy density than standard petrol. Simply put, that translates as better mileage."

Compare a Toyota Corolla 1.8-litre petrol hatchback (7.3 litres per 100km) to a Toyota RAV4 2.2-litre diesel SUV (5.8 litres per 100km); the latter is 20% more fuel-efficient than the smaller car.

"In regards the overall cost of using one versus the other, though, one has to factor in the impact of road-user charges for running a diesel vehicle," Mr Thomson said.

Assuming an average driver travels 20,000km a year, the total cost of fuel plus applicable road-user charges is as follows (worked out via the government website www.fuelsaver.govt.nz). -

• 1.8-litre petrol Corolla: $3800 per annum.

• 2-litre diesel Corolla: $2960 per annum.

• 2.2-litre diesel RAV4: $3150 per annum.

"If you crunch the dollar figures above, what you will see, pretty much, is that the cost of the road-user charge pretty much nullifies the cheaper price of diesel at the pump; basically, all of the saving you make is due to the greater fuel efficiency of diesel," Mr Thomson said.

Petrol cars are usually cheaper and cheaper to service, but a diesel will come back with a generally higher resale value.

"My general rule of thumb is that diesel makes sense if you are doing higher-than-average miles, especially a lot of open-road driving."

There are other approaches, too, Mr Thomson says.

These include. -

• Ultra-efficient petrol technology (for example, Volkswagen's fuel-stratified injection system).

• Hybrids that combine batteries and technologies such as regenerative braking, stop-start engines (Peugeot has a system that automatically stops the engine when you brake at, say, traffic lights, and restarts it).

• Partial disabling of engines (Chrysler and Holden V8s shut down four cylinders when cruising)

• Solar power to run air-conditioning.