E10 vs U91 vs U95 vs diesel: which fuel type actually saves you money?

Standing at the bowser staring at four different nozzles is a weekly ritual for most Australian drivers. E10 is cheaper per litre, but does your car use more of it? Is U95 worth the extra cost? And where does diesel fit in? The answers are surprisingly straightforward once you look at the numbers.

What the octane numbers actually mean

The number — 91, 95, 98 — is the Research Octane Number (RON). It measures how resistant the fuel is to knocking, which is premature combustion in the engine. Higher octane does not mean more power or better fuel economy in most cars. It means the fuel can handle higher compression without detonating early.

If your car is designed for U91, putting in U95 or U98 gives you almost zero benefit. You are paying more for a property your engine does not need. If your car requires U95 minimum (check the fuel filler cap or owner's manual), then U91 can cause knocking and reduced performance.

E10 is regular unleaded (U91) blended with 10% ethanol. The ethanol content slightly reduces energy density, meaning your car uses roughly 3% more fuel per kilometre. But E10 is typically 4 to 6 cents per litre cheaper than U91, which more than offsets the efficiency loss for most drivers.

Real price comparison: March 2026

Using average metropolitan prices across Sydney, Melbourne, and Brisbane as of late March 2026:

*Based on 13,300 km/yr. Petrol: 8L/100km (8.24L for E10). Diesel: 6.4L/100km. Prices are metro averages.

E10: the cheapest option for most cars

If your car is compatible with E10 — and most cars built after 2005 are — it is the cheapest petrol option. The 3% efficiency penalty is real, but at current prices you still save around $7 per year compared to U91 and $155 compared to U95. Not life-changing, but it adds up.

The catch: not every car can run E10. Check your owner's manual or the FCAI compatibility list. Older European cars and some luxury models specifically warn against ethanol blends.

Diesel: lowest cost per kilometre

Diesel engines are roughly 20 to 25% more fuel-efficient than equivalent petrol engines. A diesel Mazda CX-5 uses about 6.4 litres per 100 km compared to 7.9 litres for the petrol version. Even though diesel costs more per litre than E10 or U91, the lower consumption means diesel drivers spend less per year on fuel.

The trade-off is the higher purchase price of diesel vehicles and slightly higher servicing costs. For drivers doing more than 15,000 km per year, diesel typically breaks even within 2 to 3 years.

Premium fuel: only worth it if your car demands it

U95 and U98 cost 14 to 30 cents more per litre than U91. Over a year, that is $148 to $318 extra for a typical driver. If your car requires premium fuel — many turbocharged and European vehicles do — you have no choice. But if your car runs on U91, filling up with U98 is essentially paying $300 a year for nothing.

Some manufacturers recommend (not require) premium fuel, claiming marginal performance gains. Independent testing by the NRMA and RACQ has consistently shown the fuel economy improvement in U91 cars running U98 is negligible — typically less than 1%.

The bottom line

Check what your car actually needs. Fill with the cheapest compatible fuel. For most Australians driving a Corolla, Mazda3, or similar, that means E10 or U91. For SUV and ute owners doing higher kilometres, diesel is worth considering. And for everyone, checking prices before you fill up matters more than the fuel type you choose — because a 20-cent price swing between stations dwarfs the 6-cent difference between E10 and U91.