E40 fuel is a blend of 40% ethanol and 60% gasoline by volume. It falls into the category of “mid-level” ethanol blends, sitting between the E10 or E15 you find at most gas stations and the E85 sold at flex-fuel pumps. E40 isn’t widely available at retail yet, but it has drawn attention from researchers and ethanol advocates for its potential to improve engine efficiency and cut emissions without requiring the full leap to E85.
How E40 Differs From Regular Gasoline
The standard gasoline sold across the United States is E10, meaning it contains 10% ethanol. E40 quadruples that ethanol content. Because ethanol has a lower energy density than gasoline, each gallon of E40 contains less total energy than a gallon of E10. You might expect that to mean worse fuel economy, but the picture is more nuanced.
Ethanol has a higher octane rating than gasoline. That extra octane allows an engine to run more efficiently, partially offsetting the lower energy content. Research comparing real-world fuel use found that E25 and E40 can achieve comparable volumetric fuel economy to E10, with engine efficiency improvements of roughly 5% and 10%, respectively. In practice, this means drivers in a properly calibrated vehicle may not notice much difference at the pump.
Ethanol also has a latent heat of vaporization about three times greater than gasoline (307.3 kJ/kg versus roughly 100 kJ/kg for gasoline). When E40 evaporates inside the engine, it absorbs significantly more heat, cooling the intake charge. That cooling effect lowers combustion temperatures and can reduce the risk of engine knock, which is why higher-ethanol blends pair well with turbocharged or high-compression engines that benefit from cooler, denser air entering the cylinders.
Which Vehicles Can Use It
E40 exceeds the ethanol limit approved for standard gasoline vehicles, which are typically warrantied for E10 or, in newer models, E15. To run E40, you need a flexible fuel vehicle (FFV). FFVs are designed to operate on gasoline and any ethanol blend up to 83%. Other than an ethanol-compatible fuel system and a powertrain calibration that adjusts to different ethanol concentrations, FFVs are mechanically similar to their conventional counterparts.
Putting E40 in a vehicle not rated for it can damage rubber seals, fuel lines, and other components that aren’t designed to handle ethanol’s corrosive properties at higher concentrations. If your vehicle has a yellow gas cap or a badge indicating “Flex Fuel” or “FFV,” it can handle E40 without modification. If it doesn’t, stick with E10 or E15.
Where to Find E40
E40 is not a standard offering at most gas stations. When it is available, it comes from blender pumps (sometimes called flexible fuel pumps). These dispensers draw from two or more underground storage tanks, one holding gasoline and another holding E85, then mix them inside the pump to create intermediate blends like E20, E30, or E40. The blend ratio is selected by the customer at the pump interface.
Blender pump infrastructure has grown slowly. Stations offering mid-level blends are concentrated in corn belt states like Minnesota, Iowa, Nebraska, and the Dakotas, where ethanol production is a major industry. Regulations require that any blend above E10 be dispensed from a separate, dedicated hose, which adds cost and complexity for station owners. As a result, most retail locations that carry higher ethanol blends focus on E15 and E85, with E40 available only where blender pump technology is installed.
Emissions Compared to Gasoline
One of the strongest arguments for E40 is its emissions profile. Nitrogen oxide (NOx) emissions, a key contributor to smog, dropped by approximately 50% as ethanol content increased from pure gasoline to the E30-E40 range, with no further reduction seen at E55 or E80. That plateau suggests E40 captures most of the NOx benefit available from ethanol blending.
On the greenhouse gas side, using E40 blended with corn-based ethanol reduced total lifecycle greenhouse gas emissions by 18% compared to E10. When the ethanol was made from corn stover (the stalks, leaves, and cobs left after harvest), that reduction jumped to 32%. Non-methane hydrocarbons also decreased with higher ethanol content.
The tradeoff is that certain other emissions increase. Tailpipe levels of acetaldehyde, formaldehyde, and unburned ethanol rise with higher ethanol blends. Carbon monoxide and nitrous oxide emissions, on the other hand, showed no significant change between E10 and E40.
Performance Characteristics
E40’s higher latent heat of vaporization produces lower combustion temperatures and slower combustion velocities compared to straight gasoline. For everyday driving, this is largely invisible. The engine’s computer in an FFV adjusts fuel injection and timing to compensate. In turbocharged applications, the cooling effect can be a genuine advantage, helping prevent the pre-ignition (“knock”) that limits performance in boosted engines running on lower-octane fuel.
There is a catch, though. The same cooling properties that suppress knock can slow the rate of fuel vaporization and diffusion, making combustion slightly sluggish under certain conditions. Research on single-cylinder engines found that E40 blends decreased the constant burning rate compared to pure gasoline, a result of the fuel taking longer to fully vaporize. In a modern multi-cylinder engine with sophisticated fuel management, this effect is managed by the powertrain calibration, but it helps explain why E40 isn’t a simple drop-in upgrade for engines not designed for it.
The higher octane rating of E40 also means it could allow automakers to design engines with higher compression ratios specifically optimized for mid-level ethanol blends. Several studies have explored this idea, suggesting that purpose-built engines could extract even greater efficiency gains than current FFVs, which are compromises designed to run on everything from E0 to E85.