How fuel blending works
To hit a target ethanol content by mixing two fuels, you're solving a simple mass-balance. If fuel A is Ea% ethanol and fuel B is Eb%, the volume of A in a total V at target Et% is:
Volume A = V × (Et − Eb) ÷ (Ea − Eb), and the rest is fuel B.
The target has to sit between the two fuels' ethanol contents — you can't blend E10 and E85 into E90. Pump E85 also isn't always 85%: it's often E70–E85 depending on season and region, so measure it (or check a flex sensor) for an accurate mix.
FAQ
Why blend instead of just running E85?
More ethanol resists knock and cools the charge, but it needs more fuel flow — your injectors and pump may top out. A mid blend like E30–E50 is a popular sweet spot: much of the octane benefit while staying within stock-ish fuel-system headroom.
Is pump E85 really 85% ethanol?
Often not. In many regions it drops to E70 in winter for cold starting. If you blend assuming 85% but it's actually 70%, your real ethanol content — and your tune's fuelling — will be off. A flex-fuel sensor or an ethanol test kit removes the guesswork.