E85 Blend Calculator (E30 / E50 Mix, AFR & Injector Duty)

How it works

E85 to add = (target × total − ethanol already in tank − pump% × top-up) ÷ (E85% − pump%) · stoich AFR ≈ 14.7 − 5.7 × ethanol fraction · fuel increase = 14.7 ÷ AFR − 1 · energy per litre ≈ 1 − 0.33 × ethanol fraction

Ethanol blends are mixed by volume, so hitting a target like E30 is a two-fuel proportion problem complicated only by what is already in the tank and by the fact that pump E85 is rarely 85% ethanol. The reason people bother is that ethanol resists knock like race fuel and cools the intake charge as it evaporates, letting turbocharged engines run more boost and timing — but it carries a third less energy per litre and burns at a richer air-fuel ratio, so the injectors must flow roughly a third more at E85 and the tank empties faster. Stock fuel systems usually have headroom for E30; beyond that, pump, injectors and a tune must keep pace, and a flex-fuel sensor removes the guesswork.

Worked example

With 10 L of E10 already in a 50 L tank, reaching E30 using 75%-actual E85 means adding 15.4 L of E85 and 24.6 L of E10. The blend needs about 13% more fuel (stoich 13.0:1), pushing injectors from 70% to 79% duty — acceptable — with roughly 10% less range per tank and knock resistance around 94–95 octane plus charge cooling.

Frequently asked questions