Diesel vs Petrol: Which Actually Costs Less to Run?
Diesel is more expensive per litre, but diesel engines use less fuel. We crunch the numbers on cost per kilometre to see which really wins in Australia.
Diesel costs more per litre than petrol at the bowser. But diesel engines are more fuel-efficient, so you buy fewer litres. Which effect wins? Let's do the maths.
The price gap at the pump
In Australia, diesel typically costs 5–15 cents per litre more than U91 unleaded. The gap varies by location and market conditions — it was wider in 2022–2023 and has narrowed since — but diesel is almost always more expensive per litre.
This puts a lot of people off diesel. But price per litre is the wrong metric — what matters is cost per kilometre.
Diesel's fuel economy advantage
Diesel engines are fundamentally more efficient than petrol engines. They extract more energy from each litre of fuel thanks to higher compression ratios and the greater energy density of diesel fuel.
In practice, a diesel version of the same car typically uses 20–30% less fuel per 100km than the petrol version. For example:
| Vehicle | Petrol (L/100km) | Diesel (L/100km) | Diesel saving |
|---|---|---|---|
| Mid-size SUV | 9.0 | 6.5 | 28% |
| Small hatchback | 6.5 | 4.8 | 26% |
| Large ute | 12.0 | 8.5 | 29% |
Cost per kilometre: the real comparison
Let's compare a mid-size SUV driving 15,000 km per year:
Petrol version:
- Fuel economy: 9.0 L/100km
- U91 price: 175c/L
- Annual fuel: 1,350L × $1.75 = $2,363/year
Diesel version:
- Fuel economy: 6.5 L/100km
- Diesel price: 190c/L (15c more per litre)
- Annual fuel: 975L × $1.90 = $1,853/year
Diesel saves about $510 per year despite costing more per litre. That's almost $10 per week.
The higher the kilometres you drive, the bigger the diesel advantage. At 25,000 km/year, the saving grows to roughly $850/year.
When diesel makes sense
Diesel is likely the cheaper option if:
- You drive a lot — above 15,000 km/year, the efficiency advantage compounds
- You drive a larger vehicle — SUVs, utes, and vans show the biggest difference because they burn more fuel in absolute terms
- You do lots of highway driving — diesel engines are at their most efficient at steady highway speeds
- You tow regularly — diesel's torque advantage means less strain and less fuel under load
When petrol makes sense
Petrol is likely cheaper overall if:
- You drive under 10,000 km/year — the fuel saving is small and may not offset the higher purchase price of a diesel car
- You drive a small car — a hatchback doing 6.5L/100km on petrol already sips fuel. The diesel saving is modest in absolute dollars.
- You mostly drive short trips in the city — diesel engines need to get hot to run efficiently. Short, cold trips reduce the economy advantage and can cause diesel particulate filter (DPF) issues.
- Servicing costs matter — diesel servicing is typically more expensive (more oil, DPF maintenance, AdBlue top-ups on newer models)
The hidden costs of diesel
The cost-per-litre comparison doesn't tell the whole story. Consider:
- Higher purchase price — diesel models typically cost $2,000–5,000 more than the petrol equivalent
- Servicing — diesel services cost more, and DPF/AdBlue systems add maintenance items
- Resale — diesel cars hold value well for high-km vehicles, but the market for low-km used diesels is softer as EV uptake grows
- Emissions regulations — some cities are tightening rules on older diesels. This isn't an issue in Australia yet, but it affects long-term resale if you're keeping the car 10+ years.
The break-even calculation
To figure out your personal break-even point, you need to factor in the higher purchase price of the diesel model.
Example: If the diesel version costs $3,000 more upfront and saves $510/year in fuel, it takes about 6 years to break even on fuel savings alone. If you drive more (25,000 km/year) and save $850/year, break-even drops to 3.5 years.
If you're keeping the car for 5+ years and driving 15,000+ km/year, diesel almost always works out cheaper overall.
What about premium diesel?
Premium diesel costs another 10–15c/L on top of regular diesel. It contains additives that can improve injector cleanliness and reduce engine noise. Most mechanics agree it's a "nice to have" rather than a necessity — regular diesel is fine for the vast majority of diesel cars.
If you're curious about premium petrol, see our E10 vs U91 comparison for the petrol side of things.
The bottom line
Diesel costs more per litre but less per kilometre. For high-mileage drivers in larger vehicles, the savings are substantial — $500–1,000+ per year. For low-mileage city drivers in small cars, petrol is simpler and cheaper overall.
Either way, the biggest variable isn't which fuel type you choose — it's which station you fill up at. A 15c/L difference between two stations selling the same fuel dwarfs the diesel-vs-petrol debate.