E10 vs unleaded 91 vs diesel: what should you use?

Understand E10, unleaded 91, 95, 98 and diesel: which fuel your car needs, when premium is worth it, and how to avoid paying for octane you cannot use.

By EnergySorted Editorial Team · Updated 8 July 2026 · 8 min read

The golden rule: use what your car needs

The single most important thing about choosing fuel is that your car has a minimum requirement set by the manufacturer, and using a higher grade than it needs rarely gives you anything back. Buying 98 for a car designed to run on 91 mostly just costs more. The right question is not "which fuel is best?" but "what does my car actually require?"

You will find the answer inside the fuel filler flap, in the owner's manual, or often on the fuel cap itself. It will state a minimum octane (RON) or say the car is a diesel. Match that, and you are done. Spending more delivers little to no benefit for most everyday vehicles.

The petrol grades explained

Unleaded 91: Standard regular unleaded, 91 RON. The default for most older and many current petrol cars that do not specify higher octane.
E10: Unleaded blended with up to 10% ethanol, with an octane rating around 94. Usually the cheapest option and fine for most cars made in recent decades, but check compatibility.
Premium 95: 95 RON premium unleaded, required by some vehicles and recommended for others. Only worth buying if your car specifies it.
Premium 98: 98 RON premium unleaded, the highest common octane. Needed by some performance and European engines; wasteful for cars designed for 91.

E10: cheaper, but check compatibility

E10 is petrol blended with up to ten per cent ethanol. It is usually a few cents a litre cheaper than 91 and has a slightly higher octane rating of around 94. For most cars built in the last couple of decades it is perfectly fine, and using it is an easy way to trim fuel costs.

The catch is that some older vehicles and a few specific models are not designed for ethanol blends, where it can affect fuel-system components. Check your manual or the manufacturer's E10 compatibility guidance before switching. Ethanol also has slightly less energy per litre than pure petrol, so you may see marginally higher consumption, though the lower price usually still comes out ahead. Whichever grade you settle on, the bigger lever is buying it cheap, which is often about timing your fill around your city's petrol price cycle.

Diesel and when premium is worth it

Diesel is a different fuel for different engines: you cannot put diesel in a petrol car or petrol in a diesel without serious damage. Diesel engines are common in utes, four-wheel drives and many European cars, and tend to offer better fuel economy and torque for towing and highway driving. If your car is a diesel, that is simply what it takes; the choice is which brand and whether premium diesel is worth it, which for most drivers it is not.

Premium petrol (95 or 98) is genuinely worth it only when your car specifies it as a minimum, or recommends it and you notice a real difference. For a car that requires 91, premium will not make it faster or cleaner in any way you will feel. Using a live fuel map to find the cheapest station for the grade your car actually needs (whether that is E10, 91 or diesel) saves far more than switching grades ever will. EnergySorted's map lets you filter by fuel type so you compare like with like, and for the full set of pump-saving habits see how to find the cheapest petrol near you.

Frequently asked questions

Can I use E10 in my car?

Most cars built in the last couple of decades run fine on E10, and it is usually cheaper than 91. But some older vehicles and specific models are not designed for ethanol blends. Check your owner's manual, the fuel filler flap, or the manufacturer's E10 compatibility list before switching.

Is premium 95 or 98 worth the extra cost?

Only if your car specifies it as a minimum or recommends it and you notice a difference. For a vehicle designed to run on 91, premium delivers little to no benefit. You are mostly paying more for octane your engine cannot use.

What happens if I put the wrong fuel in?

Using a lower octane than required can cause knocking and, over time, engine strain, so match your car's minimum. Mixing up petrol and diesel is far more serious and can cause major damage. Never put diesel in a petrol car or vice versa.

Does E10 give worse fuel economy?

Ethanol has slightly less energy per litre, so you may use marginally more E10 than straight petrol. The difference is usually small, and because E10 is typically cheaper per litre, it often still works out cheaper overall for compatible cars.

How do I find the cheapest price for my fuel type?

Use a live fuel map that lets you filter by grade so you compare like with like. EnergySorted's cheapest-fuel-near-you map covers NSW and WA and lets you filter to E10, 91, 95, 98 or diesel to find the lowest price near you.

Can E10 damage my engine?

Not if your car is rated for it, which covers most petrol vehicles built in the last couple of decades. The risk is limited to older or specific models not designed for ethanol, where it can affect fuel-system components. Always check your manual or the manufacturer's E10 compatibility list first.

Is diesel cheaper to run than petrol?

Diesel engines are generally more efficient and offer better economy and towing torque, so a diesel can cost less per kilometre even when diesel is dearer per litre at the pump. But diesel and petrol are not interchangeable, and diesel prices move separately, so check the pump price for your own fuel.

What octane rating does E10 have?

Around 94 RON, slightly higher than standard 91 unleaded, thanks to the ethanol content. That is plenty for most cars specified for 91, but octane alone is not the point: compatibility with ethanol is what you need to confirm before switching.

Does when I buy matter more than which grade I buy?

Often, yes. The gap between grades is usually a few cents a litre, while the gap between the top and bottom of a price cycle can be 30 to 50 cents. Timing your fill around your city's cycle typically saves more than any grade choice.

Can I mix E10 and regular unleaded in the same tank?

For an E10-compatible car, yes. Topping up with 91 when your tank has E10 in it (or the reverse) is fine and will not harm the engine. The only real rule is never to mix up petrol and diesel, which are for entirely different engines.

Where can I find the cheapest fuel in my city?

Use a live price map filtered to your grade. For city-specific timing tips, see our guides on the cheapest petrol in Sydney and Perth.

General information only, current at the time of writing. Not financial advice. Rebate schemes and rules change; always confirm details with your retailer or state government energy site.