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frpe82
Guest
The octane rating will be very close to the blend between the fuels. 50% 91 octane gas and 50% 105 octane E85 (by mass, not volume) will yield around (0.5*91) + (0.5*105) = 98 octane. Give or take 1 or 2 octane.There's probably a precise calculation, but in general terms just assume you are mixing 91 octane fuel with 105 octane fuel, and just weight the numbers by the relative amounts and average.
The octane rating will however go up a lot with very small blends like 80% gasoline and 20% E85. That will probably give you more than the theoretical (0.8*91) + (0.2*105) = 93.8 octane. Probably closer to 95 octane.