Blending two fuels to hit a target octane is straightforward math — but "octane" in the US is the average of two different ratings (RON and MON), and E85 changes seasonally. The right blend depends on both inputs.
Octane numbers — what the pump label means
The US pump number is (R+M)/2 — the average of Research Octane (RON) and Motor Octane (MON). Europe lists RON only, which is always 4–6 points higher than the US number for the same fuel.
- US 87: ≈ RON 91 (Europe)
- US 93: ≈ RON 98
- E85 (summer blend): ≈ R+M/2 of 105–108
- Race gas (110): ≈ R+M/2 of 110
- Methanol: ≈ R+M/2 of 110, but very different combustion characteristics
Blending math
The result is a volume-weighted average:
Result Octane = (Vol₁ × Octane₁ + Vol₂ × Octane₂) ÷ Total Vol
So blending 5 gal of 93 + 5 gal of 110 → (5×93 + 5×110)/10 = 101.5 octane. Effective and cheap compared to running full race gas.
E85 seasonal warning
Summer E85 is 70–85% ethanol; winter E85 in cold-climate states drops to 51–70% ethanol to help cold start. Your octane (and AFR target!) shifts with the seasonal blend. If you're running E85 for the octane, datalog AFR after every fuel-up in winter or you'll go lean.