Mix pump 91 with E85 to hit a target ethanol content. This calculator determines how many gallons of pump E85 and pump gasoline to mix in a tank to hit a target ethanol percentage. Mustang Coyote and Modular owners running flex-fuel tunes, blended track fuel, or stepping up from 91 octane to E50 or E60 on pump gas plus E85 use it for precise mixing at the station. Enter total tank size, target ethanol percent, and measured ethanol content of each pump fuel. Keep a notebook in the glove box with your tuner's target ethanol percent and the gallon splits this tool prints for repeatability.
Mix pump 91 with E85 to hit a target ethanol content.
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This calculator determines how many gallons of pump E85 and pump gasoline to mix in a tank to hit a target ethanol percentage. Mustang Coyote and Modular owners running flex-fuel tunes, blended track fuel, or stepping up from 91 octane to E50 or E60 on pump gas plus E85 use it for precise mixing at the station. Enter total tank size, target ethanol percent, and measured ethanol content of each pump fuel. Keep a notebook in the glove box with your tuner's target ethanol percent and the gallon splits this tool prints for repeatability.
E85 pump content varies by season and station — often 70–85% ethanol, not exactly 85%. Blending wrong leaves your tune rich or lean and can damage a boosted Mustang on the edge of knock. Tuners who dial in on E60 need repeatable mixes tank to tank; guessing half tanks and splashing E85 is how a consistent Coyote becomes a stuttering mess mid-pass. Correct math keeps ethanol content aligned with your calibration file. Ethanol content also affects required fuel mass — higher blends need more flow from injectors and pumps sized on gasoline assumptions alone.
Fraction of tank as E85 = (Target ethanol% − Pump gas ethanol%) ÷ (Pump E85 ethanol% − Pump gas ethanol%). E85 gallons = Fraction × Tank gallons; Gas gallons = Tank − E85 gallons. Example: 16-gal tank, target 65% ethanol, E85 at 74%, gas at 10%: fraction = (65−10)/(74−10) = 55/64 ≈ 0.859, E85 ≈ 13.75 gal, gas ≈ 2.25 gal. Always verify with an ethanol content tester after blending — station labeling is not a lab analysis. If the fraction exceeds 1.0 or goes negative, your target ethanol percent is unreachable with the pump fuels you entered — adjust target or source different E85.
The blend ratio solves how much pump E85 and pump gas are needed to reach the target ethanol percentage.
Assuming pump E85 is exactly 85% ethanol skews every blend leaner on ethanol than intended — winter blends especially run lower. Another mistake is blending into a partially full tank without accounting for fuel already in the tank; this calculator assumes empty or you are calculating the composition of the full target volume you specify.
Many tuners start around E30–E50 on 91 octane plus E85 for added knock resistance without full flex hardware changes — exact blend depends on your tune and injector sizing. Higher ethanol supports more timing and boost on heat-soaked pulls. Use this calculator to hit the ethanol percent your tuner locked in the map, then confirm with a sensor or test tube before WOT. Flex-fuel sensor equipped S550 cars still benefit from knowing target blend math when stations sell inconsistent E85 — the sensor corrects within range but starting close saves trim work.
Use a portable ethanol tester kit or fuel composition tube from a motorsport supplier — cheap insurance before blending for a picky Coyote tune. Seasonal E85 varies; Nebraska summer E85 might test 80% while winter flex fuel drops into the 60s. Enter the measured value in this tool instead of the default 74% whenever possible. Label your jerry cans with date and tested ethanol percent — two-week-old splash mixes in the garage are a common source of inconsistent wideband readings on blended-fuel Mustang tunes.
This tool calculates a full target tank from scratch. For partial tanks, you need to account for existing gallons and their ethanol content separately — adding 5 gallons of E85 to a half tank of E10 does not equal the same result as a fresh 16-gallon blend. Drain or calculate remaining fuel math before relying on a single splash mix for a high-boost pass. Street tanks below a quarter full amplify blending error — fill strategy matters as much as the math when you are chasing a specific ethanol percent on a picky boosted tune.
Ethanol carries less energy per gallon and needs more fuel mass for the same power — higher ethanol targets increase fuel demand. Run the injector sizing calculator at your power goal with an E85-appropriate BSFC after you raise ethanol content. A Mustang that idled fine on E30 might need injector and pump upgrades before jumping to E85 on a blower car. Return-style fuel systems on Fox swaps handle high ethanol demand differently than S550 returnless rails — plan pump and injector upgrades together, not one at a time.