Of every misconception in ethanol blending, this is the one that causes the most wrong answers — because it sits underneath every calculation people make.
The specification
ASTM D5798 governs ethanol fuel blends for spark-ignition engines in the United States. It permits 51% to 83% ethanol by volume. Not 85% with a tolerance — 83% is the ceiling, in every class and every season. A pump genuinely delivering 85% ethanol is above specification. Such pumps exist, but they are uncommon: in a national survey of 91 samples, two exceeded 83%.
So what does "E85" mean?
It is a product category, the way "premium" is a product category. It tells you the fuel is a high-ethanol blend intended for flex-fuel vehicles. It does not tell you the ethanol content, any more than "premium" tells you an octane number without reading the sticker.
Why it varies
Cold-start behaviour is the main driver. Ethanol's low vapor pressure makes a high-ethanol blend hard to start in genuinely cold weather, so winter blends carry more gasoline and less ethanol. Supply and blending economics move it too. The result is real, routine variation between seasons, regions, suppliers, and sometimes between neighbouring stations. Seasonal variation in detail →
Why it matters to your calculation
Assume 85% when the pump is really delivering 74% and every blend you mix comes out low. The error compounds as your target rises: the closer your target is to the source fuel, the more sensitive the answer is to being wrong about it. At E30 an eleven-point error in the source is a nuisance; at E70 it can put the target out of reach entirely.
What to do about it
Either select a seasonal band and accept an honest range in your answer, or test your fuel and get a single number. Both are defensible. Assuming 85% is not.
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.