E85 Mix Calculator

Why E85 is a range and not a number

The number on the pump is a grade, not a measurement. ASTM D5798, the specification for ethanol fuel blends in flex fuel vehicles, permits anything from 51 to 83 percent ethanol to be sold as E85, and the blender moves within that window to hit a vapour pressure class that will still start an engine in the weather of the moment. In practice that means a summer fill is near the top of the range and a winter fill can be far nearer the bottom, and the sticker on the pump says the same thing all year. Pump gasoline has the same problem from the other end: an E10 label means up to ten percent, so a tank of it can be anywhere from E0 to E10, and E15 is sold alongside it in some markets under a waiver rather than under D5798.

That is why this calculator refuses to give you a single number. Mixing is linear, so the highest and lowest results you could really be holding sit at the corners of those label ranges, and the tool evaluates all of them and prints the two extremes next to the nominal answer. On the default case, topping 20 litres of E10 up with a fuel typed as 85 percent, the honest answer is that you are somewhere between about 14 and 29 percent ethanol, not the 30 the arithmetic promises. Anyone selling you a mixing chart that answers to a tenth of a percent is quietly assuming a fuel supply that does not exist.

The arithmetic, and the three modes

All three modes are one equation. Ethanol content of a mixture is the sum of each fuel's volume times its content, divided by the total volume, so five gallons of E85 with five gallons of E10 is (5 x 85 + 5 x 10) / 10, which is 47.5 percent. Top-up mode rearranges that to solve for the volume you have to add: the volume in the tank times the gap between the target and the tank's content, divided by the gap between the added fuel and the target. Fill mode rearranges it the other way, splitting a total you choose between two fuels. Because both are rearrangements rather than approximations, splitting a total in fill mode and then mixing those two volumes back together in mix mode returns your target exactly.

The one thing the arithmetic cannot do is reach outside the two fuels. You cannot get to 30 percent by adding E10 to a tank of E10, and you cannot get to 85 percent by topping up with E85 unless the tank is already empty, because the equation asks for a negative or infinite volume and the tool says so in words rather than printing something meaningless. Volumes are also treated as additive, which is very slightly wrong: ethanol and gasoline mixed together occupy a hair less space than the sum of the two, an effect far below the resolution of a fuel gauge and far below the label uncertainty above it.

Why more ethanol means more fuel

Ethanol carries an oxygen atom, so a kilogram of it needs less air to burn completely than a kilogram of gasoline: stoichiometric AFR is about 9.0 against 14.7. The blend's figure is a mass-weighted average of those two, computed here with exactly the function and densities that the AFR and lambda calculator uses, so the two pages can never quote different numbers for the same blend. From it come the two figures a tuner cares about: fuel mass at the same lambda scales as 14.7 divided by the blend's AFR, and fuel volume scales as that again times the density ratio, because ethanol at 0.789 kg per litre is denser than gasoline at 0.74.

Work that through for true E85 and the volume multiplier is about 1.42, while the field rule everybody quotes, and the one the injector size calculator uses, is 1.3. Both are defensible and the gap is worth understanding. Part of it is that pump E85 is not E85: at the 70 percent a real winter-to-summer average often lands on, the arithmetic gives about 1.33. Part of it is that injectors are flow-rated on gasoline and sized with a duty cycle margin on top, so 1.3 is a shopping figure rather than a fuelling figure. And part of it is that few people run identical lambda on both fuels, since ethanol's cooling and knock resistance is usually spent on more timing or more boost rather than on the same tune. The energy row in the table is the other half of the story: ethanol holds 76,330 Btu per gallon against gasoline's 116,090, which is why consumption climbs roughly in step with the volume multiplier.

What this tool will not do

It will not tell you that a blend is safe. Whether your fuel pump, lines, seals, tank coating and injectors tolerate a given ethanol content is a property of the car and its age, not of the fuel, and this page knows nothing about your car. It has no opinion on cold starting, which is the reason the standard lowers ethanol in winter in the first place, and none on water: ethanol blends hold dissolved water until they do not, and the phase separation that follows is a real failure mode that belongs in a conversation about storage and tank condition rather than in a mixing calculator. It will not recommend a content, because the right one depends on a tune, and on whether a flex fuel sensor is reading the tank for you rather than a calculator guessing at it.

Nor does it size anything. The blend's fuel volume multiplier tells you how much more fuel the engine wants, but turning that into hardware is a separate calculation with margins of its own, which is what the injector size and fuel pump tools are for. What this page does is the mixing arithmetic, plus the one thing most mixing charts leave out: an honest statement of how much of the answer is a label rather than a fact.