E85 is a flex fuel blend of 51–83% denatured ethanol and 17–49% gasoline per ASTM D5798. Despite the name, the actual ethanol percentage varies by season and supplier. E85 is sold at designated flex-fuel pumps and offers a higher octane rating (100–105 AKI) than premium gasoline.

How do I calculate the right E85 mix?

To calculate an E85 mix, you need your tank size, how much fuel is already in the tank, the ethanol percentage of that fuel, and your target ethanol blend. The formula solves for the volume of E85 and gasoline needed: E85 volume = (target% × tank − current ethanol − remaining × gas%) ÷ (E85% − gas%). This calculator handles the math automatically.

Is E85 worth it for performance?

For tuned vehicles, E85 can provide 5–20% more horsepower thanks to its higher octane rating (100–105 vs. 91–93 for premium gas) and cooler combustion temperatures. However, E85 contains about 33% less energy per gallon, so fuel economy drops 15–30%. It is worth it if your engine is tuned to take advantage of the higher octane and you have access to affordable E85.

How much horsepower does E85 add?

E85 itself does not directly add horsepower — it enables more aggressive tuning. On a properly tuned turbocharged or supercharged engine, E85 can support 5–20% more power through increased boost pressure, advanced ignition timing, and higher compression without detonation. Naturally aspirated engines typically see smaller gains of 3–8%.

Does E85 actually contain 85% ethanol?

Not always. Under ASTM D5798, E85 ranges from 51% to 83% ethanol. In winter, blenders lower ethanol content (51–70%) and add more gasoline to raise vapor pressure for cold starts. In summer, ethanol is typically higher (70–83%). The actual percentage varies by season, region, and supplier. Always test your E85 with an ethanol content tester before mixing to a specific blend target.

No. Only vehicles designated as Flex Fuel Vehicles (FFV) can run E85 from the factory. Non-FFV vehicles require upgraded fuel injectors, fuel pump, fuel lines, seals, and an E85-compatible engine tune. Running E85 in a non-flex-fuel vehicle without modifications can cause fuel system damage, lean conditions, and engine failure.

How much does E85 affect gas mileage?

E85 typically reduces fuel economy by 15–30% compared to regular gasoline. Ethanol contains about 76,000 BTU per gallon versus 114,000 BTU for gasoline — roughly 33% less energy. On partial blends like E30, the mileage impact is proportionally smaller, around 5–10% reduction.

What is the difference between E30 and E85?

E30 contains approximately 30% ethanol and 70% gasoline, while E85 contains 51–83% ethanol per ASTM D5798. E30 offers a milder octane boost (around 95–96 AKI) with less impact on fuel economy (5–10% reduction). E85 provides maximum octane (100–105 AKI) but at the cost of significantly more fuel consumption. Many tuners prefer E30–E50 as a compromise between power and economy.

How do I test the ethanol content of my E85?

Use an ethanol content tester (fuel composition sensor) or a graduated cylinder method. For the cylinder method: fill a graduated cylinder with fuel, add water equal to the fuel volume, shake, let separate, and measure the increase in the water layer — the increase represents the ethanol content. Digital ethanol testers are more accurate and cost $20–50 online.

E85 has an octane rating of approximately 100–105 AKI (Anti-Knock Index), compared to 87 for regular, 89 for mid-grade, 91 for premium, and 93 for super premium gasoline. The exact octane depends on the actual ethanol content — pure ethanol has an octane rating of about 109 AKI.

E85 Calculator

Vehicle Preset

Tank Size (gal)

Fuel Level (%)

Current Fuel Ethanol (%)

Regular pump gas is typically E10 (10%)

Target Blend

Custom Target Ethanol (%)

E85 Ethanol (%)

Gas Ethanol (%)

E85 varies 51–83% by season (ASTM D5798). Pump gas is typically E10.

Pump Gas Octane (AKI)

E85 to add for E30 blend

4.00gal

into a 15 gal tank (empty)

Blend Details

Gasoline volume, ethanol %, and octane rating

Gasoline to Add

11gal

Resulting Ethanol

30%

Estimated Octane (AKI)

Blend Reference Chart

E85 & gasoline volumes for common blends (empty tank)

Blend	E85 (gal)	Gas (gal)	Octane
E20	2	13	93
E30	4	11	95
E40	6	9	97
E50	8	7	99
E60	10	5	101
E85	15	0	106

What Is E85 Fuel?

Understanding ethanol-blended flex fuel

E85 is a fuel blend containing 51–83% ethanol and 17–49% gasoline per ASTM D5798. Despite the name, E85 rarely contains exactly 85% ethanol — the actual content varies by season, region, and supplier. In the United States, E85 is sold at flex-fuel pumps and is significantly cheaper per gallon than premium gasoline.

Ethanol Content

51–83% denatured ethanol by volume, blended with conventional gasoline

Octane Rating

100–105 AKI — significantly higher than premium gasoline (91–93)

Performance Use

E30–E50 blends are popular for street tunes balancing power and economy

When tuners refer to “E30” or “E50,” they mean a custom blend with 30% or 50% ethanol by volume. These partial blends are achieved by mixing E85 with pump gasoline to hit a specific ethanol target for a given engine tune.

How Is the E85 Mix Calculated?

The blending formula behind this calculator

The calculator solves a volume-weighted ethanol balance. Given your tank size, current fuel, and desired ethanol percentage, it determines exactly how much E85 and gasoline to add.

Blending Formula

E85 vol = (target% × tank − current ethanol − remaining × gas%) ÷ (E85% − gas%)

Solves for the volume of E85 needed to reach a target ethanol percentage, accounting for fuel already in the tank and the ethanol content of both E85 and pump gasoline.

Octane Estimation

octane = ethanol% × 109 + (1 − ethanol%) × base gas octane

Linear blending model using pure ethanol at 109 AKI and the base gasoline octane (pump octane minus the ethanol contribution already in pump gas).

Example: E30 blend in a 15-gallon empty tank

1.Target: 30% ethanol, E85 is 85%, pump gas is 10% (E10)

2.E85 = (0.30 × 15 − 0 − 15 × 0.10) ÷ (0.85 − 0.10)

3.E85 = (4.5 − 1.5) ÷ 0.75 = 4.0 gallons E85

4.Gasoline = 15 − 4.0 = 11.0 gallons gasoline

5.Octane ≈ 0.30 × 109 + 0.70 × 89 = ~95 AKI

E85 Ethanol Content by Season

Why E85 doesn't always mean 85% ethanol

Under ASTM D5798, E85 ethanol content ranges from 51% to 83%. In winter, blenders add more gasoline (lowering ethanol to 51–70%) to raise vapor pressure for reliable cold starts. In summer, ethanol content is typically higher (70–83%) since cold-start volatility is not a concern. Always test your E85 with an ethanol content tester before mixing.

Summer (Jun–Sep)

70–83%(~102–105)

Higher ethanol permitted; RVP limits cap the maximum at 83%

Winter (Nov–Mar)

51–70%(~100–102)

Lower ethanol and more gasoline added to raise vapor pressure for cold starts

Transition (Apr–May, Oct)

51–83%(~100–105)

Blend varies as stations switch between summer and winter supply

Pro tip: Use an ethanol content tester ($20–50) to measure your E85 before mixing. Enter the actual reading in the “E85 Ethanol %” field for accurate results. Winter blends can be as low as 51% in cold regions.

Benefits and Drawbacks of E85

What to consider before switching to ethanol blends

Higher octane enables more power

E85's effective octane rating of 100–105 AKI allows more aggressive ignition timing and boost on tuned engines. Gains of 5–20% horsepower are typical on flex-fuel tunes.

Cooler combustion temperatures

Ethanol's high latent heat of vaporization cools the intake charge by ~40°F more than gasoline. This reduces knock risk and lowers exhaust gas temperatures.

Reduced fuel economy

Ethanol contains about 33% less energy per gallon than gasoline (76,000 vs 114,000 BTU). Expect 15–30% lower MPG on E85. Partial blends like E30 see 5–10% reduction.

Fuel system compatibility required

E85 requires ethanol-compatible fuel lines, seals, injectors, and fuel pump. Non-flex-fuel vehicles need upgraded components and a proper tune before running E85.

Frequently Asked Questions

Common questions about E85 fuel, ethanol blending, and performance

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Last updated Apr 30, 2026