Almost every gallon of US gasoline is 10% corn ethanol — a federal mandate that quietly links pump prices to corn harvests. Here's what the blend numbers mean, the honest MPG math, and who should avoid ethanol entirely.
| Blend | Ethanol | Who it\'s for | Fuel economy |
|---|---|---|---|
| E10 | 10% ethanol | Standard US gasoline — nearly every gallon sold | ≈−3% MPG vs pure gas |
| E15 | 15% ethanol | Approved for 2001+ cars; summer sales limits partially waived | ≈−4–5% MPG |
| E85 | 51–83% ethanol | Flex-fuel vehicles only (yellow cap) | ≈−15–27% MPG |
| E0 | No ethanol | Marinas, small engines, classic cars — premium priced | Baseline |
Related: premium vs regular · RBOB (the blendstock ethanol is mixed into) · fuel cost calculator · what sets pump prices.
Federal policy. The Renewable Fuel Standard (2005/2007) mandates blending renewable fuels into gasoline — in practice about 15 billion gallons of corn ethanol a year, which works out to the standard 10% blend (E10) in nearly every US gallon. Ethanol also boosts octane cheaply, replacing additives like MTBE that contaminated groundwater.
Slightly. Ethanol carries about a third less energy per gallon than gasoline, so E10 costs roughly 3% in fuel economy versus pure gasoline, and E15 about 4–5%. E85 (51–83% ethanol) cuts MPG by 15–27% — so E85 only saves money when it's priced at least ~20% below regular.
E15 is EPA-approved for all cars from model year 2001 onward (not for motorcycles, boats or small engines). E85 requires a flex-fuel vehicle — check for a yellow gas cap, an FFV badge, or the manual. Putting E85 in a non-flex car triggers check-engine lights and, repeatedly, can damage fuel-system components.
Gasoline with no ethanol, sold at marinas and some rural stations. Ethanol absorbs water and can corrode or gum up carbureted small engines, outboards and equipment that sits idle — so boat, small-engine and classic-car owners often pay a premium for E0.