The blending formula
Mixing to a target ethanol percentage is a straight two-fuel dilution problem:
E85 gallons = tank × (target% − base%) ÷ (pump E85% − base%)
The detail most calculators get wrong is base%. Pump gasoline in the US is not 0% ethanol — it's typically E10. Treating it as zero will consistently overshoot your target, because you're ignoring ethanol that's already in the tank.
Why the pump number matters so much
"E85" is a category, not a recipe. ASTM D5798 permits 51–83% ethanol, and suppliers adjust seasonally — ethanol has poor cold-start behaviour, so winter fuel is deliberately weakened, sometimes to E51.
The consequence for blending is significant. Targeting E30 in a 15-gallon tank takes about 4.6 gallons if the pump is really delivering 75%. If it's actually delivering 55%, the same pour lands you near E25 — and if you're tuning on the assumption of E30, that gap matters. A cheap ethanol test kit removes the guesswork. More on this in your E85 isn't 85%.
Why blend down at all?
Running a partial blend rather than straight E85 is a deliberate trade, most common on tuned turbocharged cars.
- Most of the benefit, less of the cost. E30 keeps much of E85's knock resistance and charge cooling while cutting the MPG penalty roughly in half.
- Less fuel-system demand. E85 needs around 30% more fuel volume than gasoline. Blending down eases the load on injectors and pump — often the limiting factor on a stock fuel system.
- Consistency. Blending from a measured pump percentage to a chosen target is more repeatable than trusting whatever today's "E85" happens to be.
Doing it safely
Blending reduces ethanol content; it does not make ethanol safe for a car that was never built for it. You still need a flex-fuel vehicle or ethanol-compatible hardware with a supporting tune — check what cars can use E85 first. Add the E85 before the gasoline where you can, so the fuel mixes as the tank fills, and give it a drive cycle before drawing tuning conclusions.
Frequently asked questions
How do I mix E85 with regular gas to get E30?
Use the mixing formula: E85 gallons = tank size × (target% − base fuel%) ÷ (pump E85% − base fuel%). Pump gasoline is already about 10% ethanol (E10), so for a 15-gallon tank targeting E30 from 75% pump E85, you need about 4.6 gallons of E85 and 10.4 of pump gas.
Why do people blend down to E30 instead of running E85?
E30 is a common sweet spot for tuned turbocharged cars: most of the knock resistance and charge cooling of full E85, with a smaller MPG penalty and less fuel-system demand. It's also more consistent to hit reliably than trusting whatever the pump labelled E85 actually contains.
Does pump E85 really vary that much?
Yes. ASTM D5798 allows 51–83% ethanol, and blends are seasonally adjusted — winter fuel drops toward E51 so engines start in the cold. If you blend assuming 85% when the pump is actually delivering 60%, you'll land well under your target.
What octane do I get from an E30 blend?
Roughly 96–100 (R+M/2) from 91–93 premium, depending on the base fuel and the true ethanol content. The calculator estimates this by interpolating toward ethanol's ~108 effective octane. Treat it as a guide for tuning decisions, not a substitute for measuring.
Do I need a flex-fuel car to blend E85?
To do it safely, yes — either a factory flex-fuel vehicle or a car with ethanol-compatible fuel-system hardware and a tune that supports it. Blending down reduces the ethanol content but does not make E85 safe for a car that was never built for it. Check the flex-fuel list first.
Blend against your real car and your real pump
E85 App remembers your tank size and target blend, shows community-measured ethanol content for stations near you, and does the pour maths at the pump. Free on iPhone.
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