E85 calculator

Why price per gallon is the wrong comparison

E85 is almost always cheaper on the sign — typically 15–30% below regular. But a gallon of E85 doesn't take you as far. Gasoline carries roughly 114,000 BTU per gallon; E85 carries about 82,000. That's nearly 27% less energy, and your engine converts energy into distance, not dollars.

So the only comparison that means anything is cost per mile: price per gallon divided by the MPG you actually get on that fuel. A 20% discount on E85 with a 25% MPG loss means you're paying more to drive the same mile, even though the pump price looked like a bargain.

The break-even rule

There's a single number that settles it for your car:

break-even E85 price = gas price × (E85 MPG ÷ gas MPG)

If E85 is priced below that, you save. Above it, you lose — regardless of how good the discount looks. The calculator above computes this live as you change the inputs.

What MPG loss should you actually expect?

Between 15% and 27%, and where you land depends mostly on your engine.

Naturally aspirated, stock

Expect close to the full energy-density penalty, 22–27%. There's no mechanism to recover the lost energy — the ECU adds fuel to hold the air-fuel ratio and that's the end of it.

Turbocharged and tuned

Often 15–20%. E85's high effective octane and strong charge-cooling let a tune run more timing and boost, recovering some efficiency alongside the extra power. This is why E85 is popular with the tuner crowd even when the economics are neutral — you're buying knock resistance, not just fuel.

Measure it rather than guess

Run one full tank of E85, reset the trip computer, and note the MPG. Do the same on gas. Two tanks gives you a real number for your car, your right foot and your commute — far better than any published estimate. Put that figure into the calculator and the answer becomes specific to you.

What else changes the math

• Pump E85 isn't 85% ethanol. ASTM D5798 permits 51–83%, and winter blends drop toward E51. A lower-ethanol winter blend costs you less MPG than a summer blend — the actual ethanol content moves your numbers.
• Detour cost is real. E85 stations are sparser than gas stations. Driving 8 miles out of your way can erase the saving on a 15-gallon fill.
• Your driving mix matters. The MPG penalty tends to show up hardest in stop-start city driving.

Frequently asked questions

How do you calculate if E85 is cheaper?

Compare cost per mile, not price per gallon. Divide each fuel's price per gallon by the MPG you actually get on it. E85 is cheaper only when its price per gallon is discounted by more than your MPG loss — typically 15–27%.

How much MPG do you lose on E85?

Ethanol holds about 27% less energy per gallon than gasoline, and most flex-fuel drivers see a 15–27% drop in real-world MPG. Tuned turbocharged cars often land at the better end because they can exploit E85's higher effective octane; naturally aspirated engines usually see the full energy-density penalty.

What is the break-even price for E85?

Multiply the price of regular gas by your E85 MPG divided by your gasoline MPG. If you get 20 MPG on gas and 15 on E85 with gas at $3.50, you break even at $2.63 — below that E85 saves money, above it you're paying more per mile.

Is E85 worth it if I don't have a tune?

On a stock flex-fuel vehicle it's purely a price question: run the numbers above and use E85 when the discount beats your MPG loss. Tuning is what turns E85 into a performance decision as well, because the ECU can then take advantage of its knock resistance and charge cooling.

Does E85 have less energy than gasoline?

Yes. Gasoline holds roughly 114,000 BTU per gallon versus about 82,000 for E85 — near 27% less. That difference is the entire reason E85 must be meaningfully cheaper per gallon to save you money.

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