Ethanol in Gasoline: E10, E15 and E85
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.
The Blends
| 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.
When Is E85 Cheaper per Mile?
E10 at $4.33/gal (EIA week of 2026-10-12)| E85 fuel-economy penalty vs E10 | E85 price that breaks even | Discount to E10 needed |
|---|---|---|
| 15% | $3.68 | 15% ($0.65) |
| 20% | $3.46 | 20% ($0.87) |
| 25% | $3.25 | 25% ($1.08) |
| 27% | $3.16 | 27% ($1.17) |
Break-even price = E10 price × (1 − MPG penalty). The penalty range is the 15–27% quoted above; your own vehicle's figure sits somewhere inside it, so measure it over a few tanks before assuming E85 saves money. E10 here is the EIA national average for regular gasoline, which is almost all E10. Blend-by-blend detail: premium vs regular and RBOB, the blendstock ethanol is added to.
Ethanol FAQ
Why is there ethanol in gasoline?
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.
Does ethanol reduce gas mileage?
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.
Can my car use E15 or E85?
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.
What is pure gas ("E0") and why do boaters seek it out?
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.