Fuel Octane Blending (E85) Calculator
Calculate how much E85 to add to a partially filled fuel tank to achieve a target blended octane rating for performance tuning.
Results
What is it?
Octane blending allows performance tuners on flex-fuel setups to dial in a specific octane rating by mixing pump E85 with regular gasoline. Since octane blends linearly by volume, simple algebra determines how much E85 to add to an existing tank of pump gas to hit a target octane.
How to use
Enter your tank size, how much fuel is currently in the tank and its octane rating, your target blended octane, and the E85 octane at your station (check the pump or a fuel test kit). The result is the volume of E85 to add � capped at the remaining tank capacity.
Example scenario
A 60 L tank with 20 L of 95 RON pump gas, targeting 100 RON with E85 rated at 105: the formula calculates ~26.7 L of E85 to add. Adding 26.7 L to the existing 20 L leaves 13.3 L of headroom. The final blend is 46.7 L at approximately 100 RON.
Pro tip
E85 octane varies significantly by season and region (ethanol content ranges 51�83%). Use a fuel ethanol content analyzer (flex fuel sensor) for precision tuning. Always verify the blend with a refractometer or inline sensor before making aggressive timing advances � incorrect octane assumption is a common cause of engine knock damage.