Diesel vs Petrol (2026) | Running Costs, Efficiency & Best Use Cases

Fuel efficiency: diesel vs petrol

A modern diesel engine converts about 40-45% of fuel energy into motion, compared to 25-30% for petrol. This means a diesel vehicle typically uses 20-30% less fuel per 100km than an equivalent petrol model. For example, a mid-size diesel SUV might achieve 6.0L/100km while the petrol version uses 8.0L/100km at the same highway speed.

Fuel cost at the pump

Diesel is typically 5-15 cents per litre more expensive than Unleaded 91 at the pump. In 2026, Australian diesel averages around $1.75-$1.85/litre compared to $1.65-$1.95/litre for U91 depending on the cycle position. The diesel price is more stable week to week since it does not follow the weekly petrol cycle.

Running cost comparison

Assume 15,000 km/year, diesel at 6.0L/100km at $1.80/L, and petrol at 8.0L/100km at $1.90/L. The diesel annual fuel cost is $1,620. The petrol annual fuel cost is $2,280. Diesel saves roughly $660/year on fuel alone. However, diesel servicing costs $200-300 more per year on average, reducing the net saving to about $360/year. At that rate, it takes 6-14 years to recover the higher diesel purchase price.

Purchase price and depreciation

Diesel vehicles cost $2,000-5,000 more to buy new than petrol equivalents. They hold their value better in the used market for 4WDs and SUVs, but the premium is harder to justify for small hatchbacks and sedans where the fuel saving is smaller in absolute terms.

DPF and maintenance considerations

A diesel particulate filter (DPF) captures soot from the exhaust but needs regular high-temperature regeneration — typically a 20+ minute highway drive. If you mostly drive short city trips (under 10km), the DPF cannot clean itself properly and can clog, requiring a $2,000-4,000 replacement. Diesel engines also require specific low-SAPS oil and more expensive fuel filters.

Who should choose diesel

Choose diesel if you: drive 20,000+ km per year, regularly tow heavy loads like caravans or trailers, take long highway trips, or need a 4WD for rural or outback driving where diesel's torque and range offer real advantages.

Choose petrol if you: drive mostly short city trips under 15km per journey, cover less than 15,000 km per year, don't tow regularly, or prioritise a lower purchase price and cheaper servicing. For urban commuters, petrol is almost always the smarter financial choice.