PRICES & YOUR RIG
“E85” at the pump legally ranges E51–E83 by season — test it if you can.
WORTH IT?
Assumes MPG scales with fuel energy (pure ethanol ≈ 33% less than gasoline) — a solid rule of thumb, though ethanol-tuned engines often claw a few percent back. Blend price assumes you mix at the pump from the two fuels above. Octane, cooling, and power are why most people run E85 — economy is the bonus question this page answers.
HOW THE MATH WORKS
E85 is nearly always cheaper per gallon than pump gas. The honest question is cost per mile, because ethanol carries about a third less energy per gallon — a tank of E85 simply doesn't go as far. This page converts both fuels to the same yardstick: dollars per mile, using your own MPG as the baseline.
The interesting result is the break-even price. Because a blend's price and its energy content both mix linearly, the E85 price at which a blend matches pump gas per mile is the same for every target blend — E20, E30, E50, or straight E85. One threshold settles all of them: if the yellow pump is below it, every blend saves you money per mile; above it, every blend costs more and you're buying octane and knock resistance rather than economy.
Worked example: pump gas (E10) at $3.49 and a car that gets 20 MPG costs 17.5¢ per mile. True E85 carries about 74% of E10's energy, so it breaks even at $3.49 × 0.74 ≈ $2.60. E85 at $2.19? Every blend is a saver — an E30 mix runs about 17.1¢/mi and saves roughly $6 per 1,000 miles. E85 at $2.99? You're paying for performance, which is a fine thing to pay for — just know it.
Two caveats keep the numbers honest: pump "E85" legally swings from E51 in winter to E83 in summer (set the actual-ethanol slider — better yet, test it), and MPG-proportional-to-energy is a rule of thumb — ethanol-tuned engines often beat it by a few percent, which only makes the blend math look better.