E85 Fuel Calculator
Accurately calculate the exact amount of E85 and gasoline needed to achieve your desired ethanol blend.
Gallons of E85 to Add
0.00
Summary of your current fuel, what to add, and the final mix.
| Fuel Component | Volume (Gallons) | Ethanol % | Ethanol Volume (Gallons) |
|---|---|---|---|
| Current Fuel | 0.00 | 10% | 0.00 |
| E85 to Add | 0.00 | 85% | 0.00 |
| Gasoline to Add | 0.00 | 10% | 0.00 |
| Final Mix (Full Tank) | 0.00 | 0% | 0.00 |
Visual breakdown of fuel and ethanol volumes in your full tank.
What is an E85 Fuel Calculator?
An e85 fuel calculator is a specialized digital tool designed for owners of flex-fuel vehicles (FFVs) to determine the precise mixture of E85 (a blend of up to 85% ethanol and 15% gasoline) and standard gasoline (like E10) required to achieve a specific target ethanol blend in their fuel tank. Whether you’re aiming for E30, E40, or E60 for performance tuning or fuel economy purposes, this calculator removes the guesswork, ensuring you achieve the correct ratio every time you fill up. Using an e85 fuel calculator is crucial because the actual ethanol content in “E85” can vary significantly by season and location, from as low as 51% to the full 85%.
This tool is primarily for performance enthusiasts who modify their vehicle’s engine tune to take advantage of ethanol’s higher octane rating and cooling properties. By creating custom blends, drivers can optimize engine performance, but this requires great precision. An incorrect mixture can lead to poor performance, engine knocking, or even damage. Therefore, a reliable e85 fuel calculator is an indispensable utility for anyone serious about running ethanol blends.
Common Misconceptions
A primary misconception is that you can use E85 or custom blends in any vehicle. This is false and dangerous. Only vehicles specifically designated as Flex-Fuel Vehicles (FFV) have the necessary hardware (fuel pump, lines, injectors) and software to handle high ethanol concentrations. Using E85 in a non-FFV can cause severe damage. Another error is assuming the pump labeled “E85” always dispenses 85% ethanol. As our e85 fuel calculator accounts for, this percentage fluctuates. Always test the fuel or use a conservative estimate if unsure.
E85 Fuel Calculator Formula and Mathematical Explanation
The logic behind an e85 fuel calculator is based on solving a system of two linear equations with two variables. The goal is to find the volume of E85 to add (`V_e85`) and the volume of gasoline to add (`V_gas`).
1. Volume Conservation Equation: The sum of the fuel you add must equal the empty space in your tank.
V_e85 + V_gas = V_to_add
where V_to_add = TankCapacity - V_current
2. Ethanol Conservation Equation: The total amount of pure ethanol in the final mix is the sum of the ethanol from the current fuel and the fuels you add.
(V_e85 * E_e85) + (V_gas * E_gas) + (V_current * E_current) = V_tank * E_target
By solving this system, the e85 fuel calculator accurately determines the exact amounts of each fuel to add to achieve your desired outcome.
Variables Table
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
V_tank |
Total capacity of the fuel tank | Gallons | 12 – 25 |
V_current |
Volume of fuel currently in the tank | Gallons | 0 – V_tank |
E_current |
Ethanol percentage of current fuel | % | 0 – 85 |
E_target |
Desired final ethanol percentage | % | 10 – 85 |
E_e85 |
Ethanol percentage of the E85 fuel being added | % | 51 – 85 |
E_gas |
Ethanol percentage of the gasoline being added | % | 0 – 10 |
V_e85, V_gas |
Volumes of E85 and gasoline to add | Gallons | Calculated Result |
Practical Examples (Real-World Use Cases)
Example 1: Creating an E40 Blend for Performance
A driver has a Subaru WRX with a 15.9-gallon tank, tuned for an E40 blend. The tank is currently 25% full with standard E10 (10% ethanol) pump gas. The local E85 pump is known to dispense fuel with 80% ethanol content.
- Inputs for e85 fuel calculator:
- Tank Capacity: 15.9 gal
- Current Fuel Level: 25%
- Current Ethanol Content: 10%
- E85 Pump Content: 80%
- Gasoline Content: 10%
- Target Ethanol Blend: 40%
- Calculator Outputs:
- E85 to Add: 5.11 Gallons
- Gasoline to Add: 6.82 Gallons
- Final Blend: E40
- Interpretation: The driver needs to add approximately 5.11 gallons of E85 and 6.82 gallons of E10 to fill the tank and achieve the perfect E40 mix for their tune.
Example 2: Stretching a Tank of E85
A corn-belt state resident drives an FFV truck with a 22-gallon tank. They just filled up with E85 that had a true ethanol content of 70%. Now they want to dilute it down to an E30 blend to improve fuel mileage for a long trip. The tank is 90% full.
- Inputs for e85 fuel calculator:
- Tank Capacity: 22 gal
- Current Fuel Level: 90%
- Current Ethanol Content: 70%
- E85 Pump Content: 70% (Not adding more)
- Gasoline Content: 10% (This is what will be added)
- Target Ethanol Blend: 30%
- Calculator Outputs:
- E85 to Add: 0.00 Gallons
- Gasoline to Add: 15.25 Gallons (Note: this is more than the tank can hold, the calculator should flag this)
- Interpretation: The e85 fuel calculator shows it’s impossible to reach E30 simply by topping off the 2.2 gallons of empty space. The driver would need to drain a significant amount of the E70 first. This demonstrates how the calculator prevents costly mistakes.
How to Use This E85 Fuel Calculator
- Enter Tank Capacity: Input the total size of your vehicle’s fuel tank in gallons.
- Set Current Fuel Level: Provide the current fuel level as a percentage.
- Input Current Ethanol %: Specify the ethanol content of the fuel already in your tank. If you just have regular gas, this is likely 10% (for E10). If you have a partial blend, you might need an ethanol sensor to get an accurate reading.
- Set Fuel Ethanol Contents: Enter the actual ethanol percentage of the “E85” you are adding, and the gasoline you are adding. Be realistic; “E85” is rarely exactly 85%.
- Define Target Blend: Input your desired final ethanol percentage (e.g., 30 for E30, 40 for E40).
- Read the Results: The e85 fuel calculator will instantly show you the precise amount of E85 and gasoline to add to achieve your target blend upon filling the tank.
- Analyze the Chart and Table: Use the visual aids to confirm the volumes and understand the final composition of your fuel mix.
Key Factors That Affect E85 Fuel Calculator Results
- Actual E85 Content: This is the most critical variable. A difference between an assumed 85% and an actual 70% will drastically alter the required fuel amounts. Investing in an ethanol content sensor is highly recommended for accuracy.
- Starting Fuel Volume: The amount of fuel already in your tank directly impacts how much new fuel you can add, and thus affects the achievable blend.
- Current Ethanol Content: If you are starting with a partial blend, knowing its precise ethanol content is crucial for an accurate final mix. Guessing can easily throw off your target.
- Target Blend: A higher target blend (like E60) will naturally require a much larger proportion of E85 than a lower target (like E30).
- Tank Capacity: This sets the total volume for the final calculation. An accurate tank size ensures the final blend fills the tank as expected. Using a powerful e85 fuel calculator helps manage all these factors.
- Fuel Temperature and Density: While most calculators don’t account for this, fuel volume changes with temperature. For ultra-precise applications like professional racing, these factors can be considered, but for street use, the effect is minor.
Frequently Asked Questions (FAQ)
No. The output of this e85 fuel calculator should only be used for Flex-Fuel Vehicles (FFVs). Putting high-ethanol blends in a non-FFV can cause serious and expensive damage to the fuel system and engine.
If your car is tuned for a specific blend (e.g., E60) and you use a lower blend (e.g., E30), you may experience reduced performance. If you use a much higher blend, you risk engine knock (detonation), which can cause catastrophic engine failure.
The ethanol percentage is reduced in colder climates and seasons to improve the engine’s ability to start in cold weather, as ethanol is harder to vaporize than gasoline. This is why a good e85 fuel calculator allows you to input the actual content.
The most accurate method is to use a dedicated ethanol content sensor installed in your fuel line, which provides a real-time reading to a gauge or your car’s computer. Alternatively, you can use a simple test kit that involves mixing fuel and water in a graduated cylinder.
Generally, you will get worse gas mileage. Ethanol has about 30% less energy content by volume than gasoline. So, while you may gain performance from the higher octane, your fuel consumption will increase. An fuel cost calculator can help you determine the financial impact.
It is safe if done correctly. This means using an FFV, getting a professional engine tune for your target blend, and using a quality e85 fuel calculator to ensure you are mixing your fuels accurately every time.
Many performance tuners find that blends like E30-E50 offer a sweet spot, providing most of the octane and cooling benefits of E85 without requiring the massive fuel system upgrades (injectors, pumps) needed to run full E85, which demands a much higher fuel volume.
This specific calculator focuses on ethanol volume percentage, which is the primary concern for safe engine tuning. While octane will increase with ethanol content, calculating the exact final octane rating is complex and depends on the specific octane of the gasoline and E85 base fuels. See our ethanol blend calculator for more octane-focused calculations.