Quick answer
E85 is a flex-fuel blend containing 51-83% ethanol and the rest gasoline (the exact ratio varies by season and region). It has higher octane than regular gas (105+ AKI typical) but only 73% of the energy per gallon, so MPG drops 20-30% even in vehicles designed for it. E85 is safe ONLY in flex-fuel vehicles (FFVs) with reinforced fuel-system components.
What is in E85
E85 is regulated by ASTM D5798 in the US. The ethanol percentage is NOT actually 85% year-round:
- Summer (Class 1): 79-83% ethanol, 17-21% gasoline.
- Winter (Class 4): 51-70% ethanol, 30-49% gasoline. The extra gasoline is added so the fuel will vaporize and start at low temperatures.
The gasoline portion in E85 is typically a high-volatility "denaturant" specifically formulated to mix with ethanol.
Energy density and the MPG penalty
Ethanol contains 76,000 BTU/gallon. Gasoline contains 114,000 BTU/gallon. E85 (at ~75% ethanol) averages about 83,000 BTU/gallon - roughly 73% of gasoline's energy.
Real-world MPG drop in a Ford F-150 EcoBoost FFV: roughly 25-30% lower running E85 versus E10 (regular pump gas). To break even on cost-per-mile, E85 must be priced 25-30% below regular.
Why E85 has higher octane
Ethanol's research octane number (RON) is 109 and motor octane (MON) is 89, averaging an AKI of 99 for pure ethanol. E85 at the pump typically delivers 100-105 AKI.
The high octane lets tuned engines run more boost and timing without knocking. Turbo flex-fuel vehicles (F-150 EcoBoost, some Chevy SS, Polestar 2 in some markets) make significantly more power on E85 than on regular gas. This is also why E85 is popular for aftermarket tuning of turbo cars - 30-100+ extra HP is achievable with proper tuning.
Which vehicles can use E85
Only flex-fuel vehicles (FFVs) are approved for E85. FFVs have:
- Stainless steel or coated fuel lines (ethanol corrodes regular steel).
- Ethanol-compatible fuel pump seals and o-rings.
- Larger-flow fuel injectors (need ~30% more fuel volume to compensate for lower energy density).
- An ECU calibration that detects ethanol content from the O2 sensors and adjusts fueling on the fly.
Common FFVs: most Ford F-150 / Explorer / Edge (since 2006), GM trucks and SUVs with the badge, Chrysler 200 / 300 FFV, Dodge Charger / Challenger FFV, Toyota Tundra (some years), Nissan Titan (some years). Look for a yellow gas cap or "FFV" / "Flex Fuel" badge.
Damage risk in non-FFVs
Running E85 in a non-FFV causes:
- Fuel-pump and injector corrosion within months.
- Lean operation (P0171 / P0174 codes). The ECU's fuel trim cannot compensate for E85's lower energy density - the engine runs lean enough to misfire or knock.
- Damaged rubber fuel hoses and o-rings. Ethanol attacks non-ethanol-rated rubber.
- Worse MPG and rough idle.
One emergency tank of E85 in a non-FFV is unlikely to cause permanent damage. Repeated use will.
Common mistakes
- Filling up with E85 by accident. The pump nozzle is usually yellow and labeled, but they share islands with E10 / 87. Always check before pulling the trigger.
- Expecting same MPG as regular gas. A 25-30% drop is normal even in flex-fuel vehicles. Calculate cost-per-mile, not cost-per-gallon.
- Running E85 on a "tuned" non-FFV without proper tuning. Catastrophic lean conditions are possible. Aftermarket flex-fuel kits exist for this exact reason.
- Storing E85 long-term. Ethanol absorbs water aggressively. E85 stored more than 3 months can phase-separate into water/ethanol on the bottom and gasoline on top.