Mix race gas with pump gas - find the octane of a mix, or exactly how much race fuel to add to hit a target rating for a track day or aggressive tune.
Blending a few gallons of race fuel into pump gas is the classic way to buy knock headroom for a track day or a spicy tune file without running a dedicated race-fuel setup.
Gasoline-to-gasoline blends track the volume-weighted average within about a point, which is plenty for planning. The notable exception is ethanol: its effective blending octane in gasoline is higher than its rated octane, so E85 mixes outperform the linear estimate - use the E85 Blend Calculator for those, which works in ethanol percentage instead.
US pumps post (R+M)/2, also called AKI; Europe posts RON, which reads ~4-5 points higher for the same fuel (93 AKI ≈ 98 RON). Race fuels are labeled both ways depending on brand - make sure both inputs use the same scale.
Many high-octane race fuels (110+) are leaded - even small amounts permanently damage O2 sensors and catalytic converters. For street-driven cars with cats, stick to unleaded race fuels (e.g. 100-104 unleaded) or ethanol blending.
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