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FAQ

What is ethanol?

Ethanol is an alcohol produced by fermenting sugars — primarily from corn in the US, sugarcane elsewhere. It's the same alcohol found in beverages, but denatured so it's not drinkable. When blended into gasoline, it's measured as a percentage: E10 is 10% ethanol (standard pump gas), E15 (Unleaded 88) is 15%, and E85 is up to 85%.

Why is ethanol mixed with gasoline?

A few reasons:

  • The federal Renewable Fuel Standard (RFS) mandates a minimum volume of renewable fuels in the US fuel supply.
  • Ethanol has a high octane rating (~113 RON), making it an effective and clean octane booster — it replaced MTBE after that additive was banned.
  • Burning ethanol produces fewer tailpipe emissions per BTU than pure gasoline.
  • It reduces dependence on imported petroleum.

Why does ethanol hurt fuel economy?

Ethanol contains less energy per gallon than gasoline — about 76,000 BTU vs. 114,000 BTU for pure gas, roughly a 33% gap. E85 (85% ethanol) therefore has about 27% less energy per gallon than E10 under ideal conditions. Your engine burns through it faster to produce the same power, which is why you get fewer miles per gallon. E15 has only 15% ethanol, so its energy penalty is small — about 4% — and largely the same across all vehicles.

Why does vehicle type affect the E85 penalty?

Ethanol's high octane allows an engine to run more aggressive ignition timing and, on turbocharged engines, higher boost pressure — both of which extract more work from each combustion cycle. This partially offsets the energy-density disadvantage:

  • Conservative (~27% penalty) — naturally aspirated FFV with no tuning advantage. The engine treats E85 like any other fuel and takes the full energy-density hit.
  • Typical (~22% penalty) — most non-turbo FFVs. The ECU adjusts timing slightly on E85, recovering a modest amount.
  • Turbo FFV (~17% penalty) — turbocharged flex-fuel engines (e.g. EcoBoost FFV, some GM trucks) can run significantly more boost on E85, recovering the most efficiency.

What does driving style change?

City stop-and-go amplifies ethanol's disadvantage because the engine spends more time at low, inefficient loads where the ECU has less opportunity to optimize timing. Highway cruising at steady throttle gives the engine the best chance to exploit E85's octane — especially on turbo vehicles. This calculator adds 3% to the penalty for city driving and subtracts 3% for highway.

What is the winter blend?

E85 is not always 85% ethanol. In cold climates, fuel suppliers reduce the ethanol content in winter (typically November–March) to improve cold-start performance and vapor pressure. Northern-state winter E85 is often closer to E70, which reduces the energy penalty from ~27% to ~21%. If you're in a northern state and it's winter, enabling this setting adjusts the breakeven accordingly.

Why enter tank size?

The per-gallon math tells you whether a fuel is a good deal, but the per-fillup savings number is what most people find meaningful at the pump. Entering your tank size (in gallons) lets the app show how much you'd save or overpay on a full tank.