E85 Mix Calculator
Blend E85 and pump gas to hit an exact ethanol percentage — E30, E50, or anything in between — or figure out how much E85 to add to top off your tank to a target mix.
Bottom line: ethanol content adds by volume, so mixing to a target is simple math — E85 gallons = Total × (Target% − Pump%) ÷ (E85% − Pump%). Because pump E85 is seasonal and often closer to E70–E85, verify the real content with an ethanol gauge or test kit and tune to it before you lean on the numbers.
How to Use the Calculator
Pick a mode and enter your numbers:
- Blend a fresh batch: enter your target batch size, target ethanol %, your E85 content (default 83), and your pump gas content (default 10). It returns how much E85 and pump gas to combine.
- Top off my tank: enter what is already in the tank (gallons and ethanol %), your target %, your E85 content, and tank capacity. It returns how much E85 to add.
The Formula: Ethanol Content Is Additive by Volume
When you pour two fuels together, their ethanol contributions simply add up. To blend a batch to a target ethanol percentage:
- E85 gal = Total × (Target% − Pump%) ÷ (E85% − Pump%) — the E85 share needed
- Pump gal = Total − E85 gal — the remaining volume is pump gas
A target only works when it falls between your pump gas and your E85 — you cannot blend below your pump gas or above your E85.
Real-World E85 Is Seasonal
Pump E85 is rarely a true 85% ethanol. It is a seasonal blend that commonly ranges from E70 to E85, and winter blends are deliberately leaner (more gasoline) for cold starts. Content also varies station to station. A flex-fuel / ethanol content gauge or a fuel test kit beats assuming 85% — measure the actual content and plug it into the E85% field for a mix you can trust.
Higher Ethanol Needs More Fuel
- More volume: ethanol needs roughly 30% more fuel for the same air-fuel ratio, so a higher blend demands more injector and pump capacity
- Supporting mods: injectors, fuel pump, and lines must be rated for the extra flow before you raise ethanol content
- A tune: any meaningful bump in ethanol needs a matching tune (or a flex-fuel setup) — running lean on a higher blend risks engine damage
Popular Target Blends
Percentages are approximate — always verify your actual mix with an ethanol content gauge and tune to it.
| Blend | Approx. Ethanol | Why People Run It |
|---|---|---|
| E20–E30 | ~20–30% | Solid gains on stock-ish fuel systems; keeps good cold starts and drivability |
| E30 | ~30% | A popular sweet spot — meaningful power with modest fuel-system demands |
| E50 | ~50% | Bigger gains for supported setups; usually wants larger injectors and a pump |
| E70–E85 | ~70–85% | Maximum ethanol for fully built fuel systems and dedicated flex tunes |
Measure the Gain with GPS
A calculator gets your blend right; GPS timing proves it made your car faster. FastTrack measures 0-60, quarter mile, and trap speed within Dragy-class accuracy for free, so you can run back-to-back passes and quantify the delta from E30 or E50 on your actual car instead of guessing.
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Frequently Asked Questions
What ethanol percentage is E85 really?
Despite the name, pump E85 is rarely a true 85% ethanol. It is a seasonal blend that legally ranges from about E51 to E85, and cold-weather (winter) blends are leaner — often E70 to E75 — to improve cold starts. Summer blends run higher. Because content varies by station and season, an ethanol content gauge or a fuel test kit is far more reliable than assuming 85%.
How do I mix E30 or E50 from E85 and pump gas?
Ethanol content is additive by volume. To hit a target between your pump gas and E85, E85 gallons = Total × (Target% − Pump%) ÷ (E85% − Pump%), and the rest is pump gas. For a 16-gallon E30 batch from E83 and E10 pump gas, that is 16 × (30 − 10) ÷ (83 − 10) ≈ 4.4 gal of E85 and 11.6 gal of pump gas. This calculator does the math for you and warns you when a target is outside the achievable range.
Why blend instead of running straight E85?
Blending lets you dial in a consistent, repeatable ethanol level (like E30 or E50) that a stock-ish fuel system can support without swapping injectors, pumps, or lines. It also hedges against the seasonal swing in pump E85 content, and many street cars make strong, reliable gains at E30 to E50 while retaining better cold starts and drivability than full E85.
Do I need to retune for a higher ethanol blend?
Yes. Higher ethanol requires more fuel volume for the same air-fuel ratio, so any meaningful increase in ethanol content needs a tune (or a flex-fuel setup that adapts automatically) and enough fuel-system headroom — injectors, pump, and lines rated for the flow. Running lean on a higher blend without tuning risks engine damage. Verify your actual ethanol content and tune to it.
How do I measure the performance gain from an ethanol blend?
Run back-to-back timed passes before and after the blend and compare. FastTrack is a free GPS + accelerometer timer that measures 0-60, quarter mile, and trap speed within Dragy-class accuracy, so you can quantify the gain from E30 or E50 on your actual car instead of guessing.