Ethanol or Petrol? The 70% Rule for Flex Cars, and When It Is Wrong

Updated October 29, 2026

With a flex car in Brazil the question comes back at every fill-up: ethanol or petrol? The short answer is the 70% rule — if ethanol costs less than 70% of the petrol price, it pays off. The exact answer depends on your car's consumption, and sometimes it changes the result.

1. Why 70%

A litre of ethanol holds about 30% less energy than a litre of petrol, so the car goes fewer kilometres with it: if it does 11 km/L on petrol, it usually does about 7.7 km/L on ethanol. What matters for your wallet is the cost per kilometre (price per litre ÷ km per litre), and the two are even when ethanol costs 70% of petrol.

The quick sum at the pump: ethanol price ÷ petrol price. Below 0.70, fill up with ethanol; above, with petrol.

2. How high ethanol can go

Highest price per litre of ethanol for each petrol price, with the 70% rule and for a car that does 75% (common in newer engines):

Petrol	Ethanol up to (70%)	Ethanol up to (75%)
R$ 5.79	R$ 4.05	R$ 4.34
R$ 5.99	R$ 4.19	R$ 4.49
R$ 6.29	R$ 4.40	R$ 4.71
R$ 6.49	R$ 4.54	R$ 4.86
R$ 6.79	R$ 4.75	R$ 5.09
R$ 6.99	R$ 4.89	R$ 5.24

3. Your car's consumption changes the sum

The real ratio varies from car to car, usually between 65% and 75%. To find yours, divide the km/L on ethanol by the km/L on petrol. The figures are on the Inmetro label (the Brazilian vehicle labelling programme) and in the manual — or measure it yourself: fill the tank, reset the trip meter and, at the next fill-up, divide the km driven by the litres you put in.

Example: a car that does 8.1 km/L on ethanol and 11.3 km/L on petrol has a 71.7% ratio. With ethanol at R$ 4.49 and petrol at R$ 6.29 (71.4%), the 70% rule says petrol — but in this car both cost practically the same per km (R$ 0.554 and R$ 0.557). Ethanol only stops paying off above R$ 4.50.

4. How much you can save

With ethanol at R$ 4.19, petrol at R$ 6.29 and the 7.7 and 11 km/L car, driving 1,000 km a month:

Ethanol: R$ 544.16 a month;
Petrol: R$ 571.82 a month;
Saving with ethanol: R$ 27.66 a month, R$ 331.92 a year.

The closer to 70%, the smaller the difference — and the more your car's real consumption matters.

5. Other common questions

Can I mix them? Yes. A flex engine runs on any mix and the injection adjusts on its own. The cost per km falls between the two.
Do I need to switch gradually? No. After switching fuel, drive a few kilometres so the injection adapts to the new mix in the tank.
What about cold weather? Old flex cars with a small cold-start petrol tank use petrol to start on cold days; modern ones heat the ethanol and it is no longer a problem.
Does it depend on the state? A lot. In sugar-cane states ethanol often pays off; elsewhere it rarely does. The ANP (the fuel regulator) publishes a weekly price survey by state and city.

Do the maths

In the ethanol or petrol calculator, type the prices at your station and, if you like, your car's consumption: it tells you which fuel to choose, the highest ethanol price that pays off and how much you save a month and a year.

The prices in this guide are examples to show the sum; always use the ones at the station where you fill up.

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