Ethanol vs Gasoline Calculator — Which Fuel Is Cheaper per Kilometer
Every time a flex-fuel driver pulls into a station, the same question comes up: ethanol or gasoline — which one is cheaper? This calculator answers in seconds. Enter the ethanol price and the gasoline price, and it instantly tells you which fuel costs less per kilometer, using the classic 70% break-even rule (and the updated ~73% threshold for Brazil's E30 gasoline).
Flex-fuel vehicles run on any blend of ethanol and gasoline, so the choice is purely economic. The catch: ethanol is sold per liter at the same pump as gasoline, but it delivers less energy — so a lower price tag doesn't automatically make it the better buy.
How to Use the Calculator
- Enter the ethanol price per liter, exactly as shown at the pump.
- Enter the gasoline price per liter (in the US, this is E85 vs regular gasoline).
- Read the verdict — the tool compares the price ratio to the break-even point and tells you which fuel to fill up with.
- (Optional) Add your car's fuel economy on each fuel (km/L) to get the exact cost per kilometer — more accurate than the generic rule.
- Adjust the break-even point if needed: open "What is the break-even point?" and switch between the classic rule (70%) and the E30/flex rule (73%), or set your own value.
The 70% Rule Explained
The rule is simple: divide the ethanol price by the gasoline price. If the result is below 0.70 (70%), ethanol is cheaper per kilometer. If it's above, gasoline wins.
It works because ethanol carries roughly 70% of gasoline's energy per liter. One liter of gasoline yields about 8,000 kcal; one liter of ethanol, around 5,500 kcal. In practice a flex car travels about 30% fewer kilometers on ethanol — so it only pays off when its price is proportionally lower.
Why the Threshold Rises with Higher Ethanol Blends
In August 2025, Brazil raised its mandatory anhydrous-ethanol blend in gasoline from 27% (E27) to 30% (E30). Because E30 gasoline already contains more ethanol, it delivers slightly less energy itself — narrowing the efficiency gap between it and pure hydrated ethanol.
The result: the break-even point shifts from 70% to about 73%. Ethanol now pays off even at a slightly higher price. The same logic applies to E85 markets like the US and France, where real-world consumption increases of 25–30% put the break-even around 73–75%.
Examples
| Ethanol | Gasoline | Ratio | Verdict (73% rule) |
|---|---|---|---|
| 3.89 | 5.79 | 67.2% | Ethanol — below 73% |
| 4.20 | 5.79 | 72.5% | Ethanol — still pays off |
| 4.40 | 5.79 | 76.0% | Gasoline — above 73% |
| 4.23 | 5.79 | 73.0% | Essentially tied |
Edge case: when the ratio lands exactly on the threshold, the tool shows "Essentially tied" — a safe signal that either fuel is fine.
Common Mistakes
- Using the 70% rule with high-ethanol blends — with E30 (or E85), the break-even is ~73%, not 70%. The old threshold pushes you toward gasoline when ethanol is actually cheaper.
- Ignoring your car's real consumption — the rule assumes exactly 70% efficiency. Many modern turbocharged flex engines reach 73–75% on ethanol, making it competitive at higher ratios. Enter your real km/L.
- Comparing premium gasoline to regular ethanol — premium costs more. Always use the price you actually pay.
- Forgetting your mileage — for high-mileage drivers, a few cents per km becomes hundreds per month. See the impact in the fuel cost calculator for trips.
Frequently Asked Questions
Ethanol or gasoline — which is cheaper?
Divide the ethanol price by the gasoline price. With high-ethanol blends like E30, if the result is below ~73%, ethanol is cheaper per kilometer; above that, gasoline wins.
Does the 70% rule still work?
The logic holds, but the number moved. With E30 gasoline the break-even rose from 70% to about 73%. Between 70% and 73%, the old rule says "gasoline" but ethanol still pays off.
What is E85?
E85 is a fuel blend of 51–85% ethanol with gasoline, common in the US and France for flex-fuel vehicles. Because it's mostly ethanol, it carries less energy per liter — so the same break-even logic applies.
Can I change the break-even point?
Yes. Open "What is the break-even point?" and use the slider. The classic rule is 70%; the E30/flex rule is 73%. Efficient turbo engines can justify even higher thresholds.
Does ethanol harm a flex engine?
No. Flex engines are designed to run on any ethanol-gasoline ratio. The decision is purely about cost — which is why this tool focuses on cost per kilometer.
Resources
- ANFAVEA — Brazilian Vehicle Manufacturers Association — flex-fuel market share data.
- ANP — National Petroleum, Gas and Biofuels Agency — weekly fuel price surveys.
- U.S. Department of Energy — E85 Flex Fuel — E85 energy content and availability.
