Flex-fuel tuners often want a specific ethanol percentage in the tank — not just 'whatever's in there' — because ignition timing, air-fuel ratio, and boost targets all shift with ethanol content. This calculator solves the blending math so you can hit a target percentage without guesswork.

How the E85 Blend Calculator works

The calculator treats ethanol percentage as a volume-weighted average: each fuel's ethanol content is scaled by its share of the total gallons in the tank. E85 itself is assumed to be 85% ethanol for the math, since that's the standard label — though real-world E85 varies from about 51% to 85% depending on season and region (ethanol lowers cold-start performance, so blends often run lower in winter). If you know your local E85's actual content, treat the 84-85% assumption as an approximation and verify the result against a wideband ethanol sensor if precision matters.

Inputs and what they mean

Tank size sets the overflow ceiling — how many total gallons your tank can hold. Current fuel in tank and its ethanol percentage describe what's already there (E10 pump gas is the default, since that's standard at nearly every US station). Target ethanol percentage is the blend you're solving for. The target has the largest effect on the result: small increases near the top of the range (approaching 85%) require disproportionately more E85, since the equation's denominator (85 − targetPct) shrinks toward zero.

Limits and edge cases

You cannot reduce ethanol percentage by adding E85 — if your target is at or below the current fuel's ethanol %, the calculator flags it as invalid rather than showing a negative gallon amount; drain some fuel first if you need a lower blend. Targets at or above 85% are also rejected, since the math assumes E85 tops out at that level. When the solved amount would overflow the tank, the calculator instead reports the maximum E85 you can add and the highest percentage that's actually achievable. For race fuel or dedicated E85 tuning where cold-start behavior matters, cross-check the final blend with an in-tank ethanol content sensor rather than relying on pump labels alone.