Blending E85 with premium pump gas lets you dial in a precise ethanol target — say E30 or E50 — for higher octane without committing to a full tank of E85. It's a staple of ethanol tuning because E85's ~100–105 octane is cheap power, but full E85 isn't always what a given tune wants.
The catch most guides miss
Pump E85 is not a fixed 85% ethanol — it's legally 51–83% and drifts by season and region. So a fixed "X gallons E85 + Y gallons premium" recipe will miss your target whenever the pump's actual ethanol content isn't what you assumed. To blend accurately, you need the real ethanol percentage of the E85 you're using. (More: your E85 probably isn't 85%.)
The math
To hit a target ethanol fraction T, blending E85 of actual ethanol E with premium of ethanol P (pump premium is ~10% ethanol / E10), the fraction of your fill that should be E85 is:
E85 fraction = (T − P) ÷ (E − P)
Example: target E30, pump E85 measured at 70%, premium at 10% (E10): E85 fraction = (0.30 − 0.10) ÷ (0.70 − 0.10) = 0.33. So a 14-gallon fill is ~4.7 gal E85 + ~9.3 gal premium.
The easy way
Doing that at the pump is error-prone, so it's built into E85 App's blend calculator: set your tank size, target ethanol %, the real pump E85 %, and your premium octane (91/93), and it returns the exact gallons to pour plus the octane you'll land on. Free on iPhone.
Nail your blend every fill
The blend calculator does the math for your exact tank and target. Free on iPhone.
Download on theApp StoreRelated: Your E85 isn't 85% · Find E85 near you