Mixing E85 with 93 octane
This is the most common blending question there is, and the one most often answered wrong —
because the "93 octane" side of the equation is not ethanol-free.
Your 93 is almost certainly E10
Most pump premium in the United States contains up to 10% ethanol, realistically around 9.8%.
If a calculator treats it as E0, every answer it gives you overshoots. The error is largest at
low target blends, where gasoline makes up most of the mixture.
Octane is deliberately not shown
People often reach for this calculation hoping for a resulting octane number. We do not
display one, on purpose. Ethanol's blending octane is non-linear and depends on the base fuel —
it behaves like roughly 152-156 RON at low blend percentages, not the 109 of neat ethanol.
Straight-line interpolation by volume is provably wrong, and several shipping calculators that
display octane are demonstrably incorrect.
The full reasoning →
What we show instead
The stoichiometric air-fuel ratio for the resulting blend, which is a real, derivable number
and the one that actually matters to a calibration. For E50 it is 11.76:1.
Why this calculator is different
Most E85 calculators assume pump E85 is exactly 85% ethanol. It almost never is. ASTM D5798
permits anywhere from 51% to 83% ethanol, and what's actually in the pump varies by season,
region, and station.
It tells you the truth about precision
If you tell us the pump content is a range, your answer comes back as a range. Five gallons of
unknown pump E85 into ten gallons of E10 lands anywhere between E23.5 and E34.2 — showing you a
single number there would be a lie dressed up as accuracy.
It shows what the rounded amounts actually give you
You can't pump 8.5714 gallons. So we also tell you what you'll really end up with after you
round to something a pump can deliver — 8.6 and 6.4 gallons gives you E50.13, not E50. No other
calculator we tested reports this.
It never says "invalid"
If your target can't be reached, you get the reason and your actual options — drain this much,
or burn down to this level, or here's the closest you can get. An impossible request is still a
real situation you're standing in.
It deliberately does not show octane
Ethanol's blending octane is non-linear and depends on the base fuel — it behaves like roughly
152–156 RON at low blend percentages, not the 109 of neat ethanol. Straight-line interpolation
is provably wrong, and of the calculators we reviewed that display octane, several are
demonstrably incorrect. A wrong number with a disclaimer is still a wrong number.
Read the full reasoning →
Frequently asked questions
How much E85 do I need to make E50?
It depends on what's actually in the pump and what gasoline you're mixing with. With E80 pump
fuel and E10 gasoline in a 15-gallon tank, roughly 8.6 gallons of E85 and 6.4 gallons of 93.
Use the calculator above with your real numbers.
Is pump E85 always 85% ethanol?
No. ASTM D5798 allows 51–83% ethanol in every class and season. Warm-climate states typically
run near the top of that range year round; cold-winter states drop lower in winter for
cold-start reasons. Nothing labeled E85 is required to contain 85% ethanol — in fact the
standard caps it at 83%.
Can I mix E85 with 93 octane?
Yes, that's the most common way to reach a target blend like E30 or E50. The calculator handles
the gasoline's own ethanol content — most pump 93 is E10, around 9.8% ethanol, which shifts
the math more than people expect.
What if my ethanol percentage is too high?
Choose "Fix my current blend." If there's room in the tank you can simply add gasoline. If the
tank is full, the calculator tells you how much to drain, or how far to burn it down first,
and what each option gets you.
Does E85 require a tune?
For a non-flex-fuel vehicle, yes — ethanol needs substantially more fuel volume than gasoline,
and running it without a calibration that accounts for that risks a dangerously lean condition.
Flex-fuel vehicles handle it natively. This calculator does not confirm whether your vehicle
or fuel system can handle a given blend.
Can I use the calculator in liters?
Yes — the gal/L toggle switches every volume. The engine works in liters internally, so
there's no conversion drift either way.
Will it work without cell service?
The calculator runs entirely in your browser with no network calls, so once the page has
loaded it keeps working with no signal. The station finder will need a connection for live
data.
Why Palm Beach Dyno built this
Palm Beach Dyno tunes high-performance vehicles on gasoline, ethanol blends, and E85 — in-shop
on the dyno and remotely. Accurate blending matters to us because pump E85 varies in real
ethanol content, and a vehicle with a performance calibration may require a specific blend to
run the way it was tuned to run.
This calculator uses the same tested calculation engine as the one inside our TechCenter, which
our own staff use on customer vehicles. It's checked against 276 assertions and a set of golden
test vectors generated from an independent exact-arithmetic reference implementation — not from
the code being tested.
E85 and flex-fuel tuning → ·
PBD TechCenter →