E10 is usually the cheapest fuel on the price board. But cheaper per litre doesn’t always mean cheaper per kilometre. Here’s the arithmetic to check.

What E10 is

E10 is a blend of 90% regular unleaded petrol and 10% ethanol. Most petrol cars sold in Australia since the early 2000s can run on it, but always check your car’s manual or fuel cap first.

Why E10 uses more fuel

Ethanol holds less energy than petrol. According to the NSW Government’s own guidance on E10, the blend has “around 3% less energy than the equivalent amount of RON 91 petrol.” That means a car burns roughly 3% more E10 than 91 to cover the same distance.

This isn’t a flaw in your engine. It’s a property of the fuel itself, in the same way driving on under-inflated tyres uses more fuel for the same trip.

The break-even rule

Because E10 uses about 3% more fuel, it only saves you money if it’s priced at least 3% cheaper per litre than 91. If the discount is smaller than that, E10 can end up costing more per kilometre, even though it looks cheaper at the pump.

Rule of thumb: required discount (cents per litre) ≈ 3% of the 91 price.

A worked example

Say your car uses 8.0 litres per 100 km on unleaded 91.

On E10, consumption rises by about 3%:

  • 8.0 L × 1.03 = 8.24 L per 100 km on E10

Now compare two prices, both illustrative, not live figures. Always check current prices with NSW’s FuelCheck tool before you buy.

  • 91 at $1.90 per litre: 8.0 L × $1.90 = $15.20 per 100 km
  • E10 at $1.85 per litre: 8.24 L × $1.85 = $15.24 per 100 km
Cost to drive 100 km on 91 versus E10, in the guide's worked example $15.20 91 8.0 L × $1.90 $15.24 E10 8.24 L × $1.85
In the guide's example, 91 costs $15.20 and E10 costs $15.24 per 100 km, because E10 needs about 3% more fuel to cover the same distance.

In this example, E10 is 5 cents per litre cheaper, but it still costs slightly more per 100 km. That’s because the break-even discount here is about 5.7 cents per litre (3% of $1.90), and the actual gap was only 5 cents.

If E10 had been priced at $1.84 per litre or lower, it would have won on cost per kilometre.

How to use this yourself

  1. Check both prices for the day, ideally at the same station, using a live tool like FuelCheck.
  2. Multiply the 91 price by 0.97. That’s the E10 price you need to beat.
  3. If E10 is priced below that number, it’s the cheaper choice for that tank. If not, 91 is likely cheaper per kilometre, even at a higher price per litre.

The takeaway

E10 isn’t automatically the money-saving choice just because it’s cheaper per litre. Do the 3% check before you decide, especially when the discount looks small.

Source: NSW Government: E10 facts