E85 vs. Regular Gasoline: Understanding the Chemical Differences

When you pull up to a gas pump and see E85 next to regular gasoline, you’re looking at two very different fuels. While both will power an internal combustion engine, their composition, properties, and performance characteristics vary significantly. Understanding these differences can help you make informed decisions about which fuel is right for your vehicle. Let’s dive deep into what makes E85 distinct from regular gasoline.

What’s in Your Fuel? Breaking Down the Composition

Regular Gasoline (E10)

Most gasoline sold in the United States isn’t pure gasoline—it’s actually already an ethanol blend. When you fill up with “regular” gas, you’re typically getting E10, which consists of:

• **90% gasoline** (petroleum-derived hydrocarbons)
• **10% ethanol** (corn-based alcohol fuel)

Note that most gasoline sold in the U.S. has some amount of ethanol blended in, usually up to a maximum of 10 percent. So you’re already putting some amount of ethanol into your vehicle’s gas tank without necessarily realizing it.

The ethanol in regular gas serves several purposes:

• Increases octane rating
• Adds oxygen to the fuel for cleaner combustion
• Reduces emissions
• Stretches the gasoline supply with renewable content

E85 (Flex Fuel)

E85 is an ethanol-heavy blend with a dramatically different composition. Despite its name suggesting 85% ethanol, the actual blend varies:

• **51% to 83% ethanol** (depending on season and region)
• **17% to 49% gasoline**

In the United States, the exact ratio of fuel ethanol to hydrocarbon may vary according to ASTM 5798, which specifies the allowable ethanol content in E85 as ranging from 51% to 83%.

Why the Ethanol Content Varies

The variation in E85’s ethanol content isn’t arbitrary—it’s carefully calibrated for different conditions:

Summer Blends (Higher Ethanol):

• Contain 70-83% ethanol
• Easier to vaporize in warm weather
• Maximum environmental benefits
• Better performance characteristics

Winter Blends (Lower Ethanol):

• Contain 51-70% ethanol
• Lower ethanol content aids cold-weather starting
• Prevent performance issues in freezing temperatures
• May be labeled E70 or E75 but still sold as “E85”

The lower heating value of neat ethanol makes it difficult to start engines in relatively cold climates without pre-heating air intake, faster cranking, or mixing varying fractions of gasoline. This is the primary reason ethanol fuel is blended with gasoline rather than sold as pure E100 in cold climates.

Other Ethanol Blends

For context, here’s the full spectrum of ethanol-gasoline blends:

• **E0**: Pure gasoline with no ethanol (rare, typically only at specialty stations)
• **E10**: 10% ethanol, 90% gasoline (standard “regular” gas)
• **E15**: 15% ethanol, 85% gasoline (approved for 2001+ vehicles)
• **E30-E50**: Mid-range blends (sometimes used by enthusiasts)
• **E85**: 51-83% ethanol (flex fuel vehicles only)
• **E100**: Pure ethanol (used in Brazil, not available in U.S.)

Chemical Properties: How E85 Differs from Gasoline

Molecular Structure

Ethanol (C₂H₅OH):

• An alcohol with a hydroxyl (-OH) group
• Contains oxygen atoms within the molecule
• More polar than gasoline hydrocarbons

Gasoline:

• A mixture of hydrocarbons (primarily C₄-C₁₂)
• No oxygen in molecular structure
• Nonpolar liquid

This fundamental chemical difference creates cascading effects on fuel properties and performance.

Energy Content (BTU)

One of the most important differences is energy density:

Regular Gasoline (E10):

• 114,000–120,000 BTUs per gallon
• Higher energy density means more power per gallon

E85:

• 83,950–95,450 BTUs per gallon
• Approximately 75% of gasoline’s energy content
• Lower energy density requires more fuel for same power

E85 contains approximately 75% of gasoline’s energy per gallon. This difference explains the MPG reduction—you need to burn more E85 to generate the same amount of energy. However, comparing energy expressed as heat value doesn’t reflect the total work from an engine cycle, as energy content doesn’t factor in certain combustion advantages ethanol provides.

Octane Rating

Regular Gasoline:

• 87 octane (regular)
• 89-90 octane (mid-grade)
• 91-93 octane (premium)

E85:

• 100-105 octane (some sources cite up to 108)
• Significantly higher than even premium gasoline
• Allows for higher compression ratios

E85 has an octane rating higher than that of regular gasoline’s typical rating of 87, or premium gasoline’s 91-93. This allows it to be used in higher-compression engines, which tend to produce more power per unit of displacement than gasoline counterparts.

The high octane is due to ethanol’s molecular structure—the oxygen atom and chemical bonds make it more resistant to premature ignition (knock).

Stoichiometric Air-Fuel Ratio

Gasoline:

• 14.7:1 (14.7 parts air to 1 part fuel)
• Engines designed around this ratio

Ethanol:

• 9.0:1 (9 parts air to 1 part fuel)
• Requires more fuel per unit of air

This is why flex fuel vehicles need larger fuel injectors—they must deliver about 34% more fuel volume when running on E85 compared to gasoline. The onboard computer detects ethanol content and adjusts fuel injection accordingly.

Oxygen Content

Gasoline:

• 0% oxygen content
• Burns with oxygen from intake air only

Ethanol:

• 35% oxygen by weight
• Carries its own oxygen, improving combustion

Ethanol has a higher oxygen content, which helps the engine burn fuel more efficiently. This built-in oxygen acts somewhat like a “chemical supercharger,” similar to nitrous oxide or nitromethane, allowing more complete combustion and potentially higher power output.

Physical Properties and Behavior

Vapor Pressure and Volatility

E10 Gasoline:

• More volatile (evaporates more easily)
• Higher vapor pressure
• More evaporative emissions
• Easier cold starting

E85:

• Less volatile than gasoline
• Lower vapor pressure
• Fewer evaporative emissions (better for air quality)
• Can be harder to start in cold weather

E85 is less volatile than gasoline or low-volume ethanol blends, which results in fewer evaporative emissions. This makes E85 better from an environmental and air quality perspective.

Corrosiveness

Gasoline:

• Relatively non-corrosive to metals
• Compatible with standard rubber and plastics
• Stable in traditional fuel systems

Ethanol:

• More corrosive, especially to certain metals
• Can degrade some rubber hoses and gaskets
• Requires ethanol-resistant materials (hence FFV modifications)

This is why flex fuel vehicles have stainless steel fuel lines, sometimes lined with plastic, and stainless-steel fuel tanks in place of traditional tinplate tanks. The elimination of bare magnesium, aluminum, and certain rubber parts in the fuel system is necessary for E85 compatibility.

Water Absorption

Gasoline:

• Hydrophobic (repels water)
• Water contamination causes phase separation
• Less prone to moisture absorption

Ethanol:

• Hygroscopic (absorbs water from air)
• Can tolerate some water content
• Can cause issues if too much water accumulates

E85 is not as sensitive toward water as low-level ethanol/gasoline blends like E10, but phase separation may still occur, particularly with intermediate blends and at low temperatures. This is why E85 shouldn’t be stored for extended periods without use.

Solvent Properties

Gasoline:

• Good solvent for petroleum-based deposits
• Doesn’t clean as aggressively
• Can leave carbon deposits over time

Ethanol:

• Excellent solvent for many deposits
• Actively cleans fuel system components
• Removes carbon buildup and varnish

Ethanol’s solvent properties mean E85 cleans your fuel system while you drive. This is generally beneficial, though it can initially dislodge accumulated deposits in older vehicles, potentially clogging filters.

Combustion Characteristics

Flame Temperature

Gasoline:

• Higher combustion temperature
• More heat stress on engine components

Ethanol:

• Cooler burning (lower flame temperature)
• Less thermal stress
• Better for turbocharged applications

Ethanol requires lower combustion temperatures and creates a thermodynamic cooling effect. This cooling benefit helps engines run cooler and can prevent valve burning, extending engine life.

Ignition Properties

Gasoline:

• Autoignition temperature: ~495°F (257°C)
• Easier to ignite when cold
• Lower octane leads to more knock tendency

Ethanol:

• Autoignition temperature: ~685°F (363°C)
• Harder to ignite when cold (hence winter blends)
• Much more knock-resistant

This is why E85 performs so well in high-performance applications—it resists knock even under high boost or compression.

Emissions Profile

Both fuels produce emissions, but the composition differs:

Gasoline (E10) Emissions:

• Higher carbon monoxide
• Contains benzene (carcinogen)
• More aromatic hydrocarbons (PAHs)
• Higher greenhouse gas impact
• More particulate matter

E85 Emissions:

• 46% lower CO₂-equivalent greenhouse gases
• No benzene content (ethanol itself lacks aromatics)
• Lower carbon monoxide
• Potentially higher acetaldehyde (in some conditions)
• Cleaner burning overall

E85 can further reduce emissions of certain pollutants compared to conventional gasoline. For example, E85 is less volatile than gasoline, which results in fewer evaporative emissions. Using E85 also reduces carbon-monoxide emissions and provides significant reductions in emissions of many harmful toxics, including benzene, a known human carcinogen.

Performance Implications

Power Output

Despite lower energy content, E85 can produce more power than gasoline when engines are properly optimized:

Why E85 Makes More Power:

• Higher octane allows more aggressive ignition timing
• Cooling effect allows higher compression or boost
• Built-in oxygen improves combustion efficiency
• Can run richer air-fuel ratios safely

As more effort is put into maximizing an engine to take advantage of E85’s higher octane rating, engines achieve greater power advantages. Some vehicles produce 10-20% more horsepower on E85 than gasoline—the Koenigsegg CCXR, for example, produces 20% more horsepower on E85 than on gasoline.

Fuel Economy

The trade-off for E85’s benefits is reduced fuel economy:

E10 Gasoline: Baseline MPG (100%)

E15: 4-5% lower MPG

E85: 15-27% lower MPG (depending on blend and vehicle)

The fuel economy difference between using E85 and gasoline depends on the ethanol blend and vehicle: the lower bound (15% MPG decrease) is based on the energy content difference of a 51% ethanol blend, while the upper bound (27% MPG decrease) comes from official EPA tests of FFVs operating on E85 versus gasoline.

Cost and Availability

Production Costs

Gasoline:

• Derived from crude oil refining
• Price tied to global oil markets
• Dependent on geopolitical factors
• Subject to refining capacity constraints

Ethanol:

• Made from domestic corn (primarily in U.S.)
• Price tied to agricultural commodity markets
• Benefits from federal subsidies and tax credits
• Production capacity continues to expand

The sourcing and composition of ethanol used in E85 fuel, combined with government subsidies for corn producers, helps keep E85 costs lower than gasoline.

Infrastructure

E10 Gasoline:

• Available at nearly all gas stations
• Standard infrastructure nationwide
• Compatible with all distribution systems

E85:

• Available at approximately 3,900-4,500 stations
• Concentrated in Midwest corn-growing states
• Requires specialized pumps and storage
• Limited in coastal and New England regions

Which Fuel Is Right for Your Vehicle?

For Non-Flex Fuel Vehicles:

• **E0 or E10**: Standard choice, widely available
• **E15**: Available option for 2001+ vehicles
• **E85**: Never use—will cause problems

For Flex Fuel Vehicles:

• Can use any blend from E0 to E85
• E85 offers environmental and potentially performance benefits
• Gasoline provides better fuel economy
• Choice depends on price, availability, and priorities

The Bottom Line: Two Fuels, Different Purposes

E85 and regular gasoline are fundamentally different fuels with distinct characteristics:

E85 Advantages:

• Much higher octane (100-105 vs. 87-93)
• Cleaner emissions (46% less GHG)
• Renewable, domestic production
• Excellent engine cleaning properties
• Cooler combustion
• Potential for more power

E85 Disadvantages:

• 25% lower energy density
• 15-27% worse fuel economy
• Limited availability
• Only for flex fuel vehicles
• Cold-start challenges
• Requires ethanol-compatible components

Regular Gasoline Advantages:

• Higher energy density
• Better fuel economy
• Universal availability
• Works in all gasoline vehicles
• Easier cold starting
• Longer driving range per tank

Regular Gasoline Disadvantages:

• Lower octane
• Higher emissions
• Petroleum-based (not renewable)
• Contains harmful aromatics
• Greater dependence on foreign oil

Understanding these chemical and practical differences helps explain why E85 isn’t simply a “cheaper substitute” for gasoline—it’s a distinctly different fuel with its own characteristics, requiring specially designed vehicles to use it safely and effectively. For flex fuel vehicle owners, the choice between E85 and gasoline comes down to balancing priorities: environmental impact, performance, cost, and convenience.

The good news? If you own an FFV, you have the flexibility to choose the fuel that best meets your needs at any given time, taking advantage of E85’s benefits when it makes sense, and falling back on gasoline when necessary. That versatility is the true advantage of flex fuel technology.