E85 is not a fixed formulation. It is a fuel specified by a range, and where a given delivery falls inside that range depends on when and where it was blended.
What the standard actually says
ASTM D5798 defines ethanol fuel blends for spark-ignition engines. It permits 51% to 83% ethanol by volume in every class and every season. The classes differ in vapor pressure, not in ethanol limits — but vapor pressure is exactly why ethanol content moves.
Why winter fuel has less ethanol
Ethanol has a low vapor pressure relative to the light hydrocarbons in gasoline. A high-ethanol blend in genuinely cold weather can struggle to produce enough vapor for a reliable cold start. Blenders compensate by increasing the gasoline fraction in winter, which raises vapor pressure and lowers ethanol content — which is why the same pump can hand you fuel in the low 60s in February and the high 70s in July.
Why Florida is different
Warm-climate states never get the cold-start problem, so they never get the seasonal reduction. Gulf Coast E85 tends to sit at the top of the range year round. This is the single most important reason the calculator will never infer a seasonal band from the date or your location: a calendar-driven default would hand our own market a band that does not apply to it. You select the band, always.
Reasonable assumptions
| Situation | Assume |
|---|---|
| Warm climate, year round (FL, Gulf Coast) | 74-83% |
| Cold-winter state, roughly May-September | 74-83% |
| Cold-winter state, roughly October-April | 62-80% |
| No idea | 51-83% (full legal range) |
These are the bands built into the calculator. They are derived from the ASTM limits and from Coordinating Research Council survey work — CRC E-85-2 measured a summer-class mean near 80% and a winter-class mean near 74%; CRC E-85-3 measured 77% across classes 1-3 and 71% in its coldest class.
An honest limitation
The most recent national E85 composition survey we can find sampled fuel in 2014. We have searched for newer work and have not found any. That makes this data roughly a decade old, and it is the main reason the bands are deliberately wide rather than precise. Treat them as reasonable priors, not measurements — and if the exact number matters to your car, measure it. How to test ethanol content →
Written by the Palm Beach Dyno team — a Ford performance shop running dyno and remote calibration on gasoline, ethanol blends and E85. Last reviewed 4 August 2026.