Compare the real cost of gasoline and ethanol using the 70% rule and the cost per kilometre with your car's actual mileage.
The 70% rule is a shortcut: if the price of ethanol is 70% or less of the price of gasoline, ethanol is cheaper to drive on. The rule comes from the energy difference between the two fuels — a litre of ethanol delivers roughly 70% of the energy of a litre of gasoline, so a car that does 12 km/L on gasoline typically does about 8.4 km/L on ethanol. When the price ratio is exactly 70%, the cost per kilometre is the same; below it ethanol wins, above it gasoline wins.
The shortcut works for most flex-fuel cars because the mileage ratio tends to hover around that 70% mark. But it is a population average, not a measurement of your car. Engine tuning, driving style, city versus highway, air conditioning and tyre pressure all move the real ratio. This calculator lets you type your actual mileage on each fuel and see the cost per kilometre, which is the number that actually determines which pump saves you money. If you do not know your mileage, the default values are a reasonable starting point for a mid-size sedan.
Everything runs in your browser and nothing is stored. The prices and mileages you type are not uploaded or logged.
Price ratio = ethanol price ÷ gasoline price. If ≤ 0.70, ethanol is cheaper (by the rule of thumb). Cost per km = fuel price ÷ mileage. Trip cost = distance ÷ mileage × price.
From the energy content of the fuels. A litre of hydrous ethanol (E100) delivers about 70% of the energy of a litre of gasoline (E27 in Brazil). The ratio translates directly into mileage for a well-tuned flex engine.
No. It is a population average. Your car might do 75% or 65% of the gasoline mileage on ethanol, depending on the engine, the tune and how you drive. The cost-per-km calculation with your real mileage is always more accurate than the ratio alone.
The one you measured in your own car, on the kind of driving you actually do. Fill the tank, reset the trip meter, drive until the next fill, and divide the kilometres by the litres. Do it twice and average.
Not in a flex-fuel car — the engine is designed for both fuels and adjusts the ignition timing automatically. In a gasoline-only car, sustained use of ethanol can damage rubber seals and fuel lines.
No. The calculation runs entirely in JavaScript on your device.
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