Blending pump E85 with premium gasoline is how most people reach a target like E30 or E50. The arithmetic is simple. The inputs are where it goes wrong.
Your 93 is not ethanol-free
Most pump premium in the United States contains up to 10% ethanol — realistically about 9.8%. That ethanol counts toward your blend, and ignoring it makes every answer overshoot. The error is largest at low targets, where gasoline is most of the mixture.
If you have access to genuine ethanol-free gasoline, select E0 in the calculator. Do not assume it — in most of the country, premium is E10.
Neither is your E85 what the sign says
Same problem from the other direction. Assume 85% when the pump delivers 76% and you land well short of your target. Between the two errors, an "obvious" half-and-half fill for E50 commonly lands nearer E45.
Order and mixing
Splash blending in the tank works, with two practical caveats. Put the larger volume in first where you can — it mixes more thoroughly. And a freshly filled tank is not immediately homogeneous. Ethanol and gasoline are fully miscible and will combine, but give it a few minutes and some driving before assuming a content sensor reading is representative.
Getting the amounts right
Pumps dispense in tenths of a gallon, so use the rounded figures — and pay attention to what the calculator tells you those rounded figures actually produce. It is usually within a fraction of a percent of your target. Occasionally it is not, and you should know which.
What about the resulting octane?
We deliberately do not display one. Ethanol's blending octane is non-linear and depends on the base fuel, so a linear estimate is wrong — and several calculators that show octane are demonstrably incorrect. The full reasoning →
A safety note that is not boilerplate
Increasing ethanol content in a vehicle that is not calibrated for it is not a neutral change. It alters the fuel volume the engine requires. Follow the requirements of your specific tune, and if you do not know what those are, ask whoever calibrated it before you change fuel.
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.