How to Mix E85 Correctly
TL;DR — Quick Summary
- Formula: (Target% - Current%) / (E85% - Target%) = Ratio of E85 to Gas.
- Safe Blend: E30 (30% Ethanol) is the safest limit for most stock fuel systems.
- Winter Warning: E85 is often E70 in winter; adjust your math accordingly.
E85 mixing is the process of combining E85 (85% ethanol) with regular gasoline to achieve a precise ethanol content, such as E30 or E50. This is done to increase fuel octane and cooling properties for high-performance tuning without exceeding the flow limits of a stock fuel system.
common practice for performance enthusiasts. Here is the science behind the mix.
The Formula
The calculation is a weighted average of the ethanol content in two liquids:
Step-by-Step Guide
- Know your Tank Size: Check your manual. Precision matters.
- Test the Pump E85: According to AFDC, "E85" at the pump is rarely 85%. It can legally range from 51% to 83%. In winter, it is often E70.
- Use a Calculator: Don't do this in your head. Use our Mixing Calculator to get the exact gallons.
- Pump Gas First: It mixes better if you add the lower density fuel first (optional, but recommended).
- Drive to Mix: It takes a few minutes of driving for the fuel to homogenize and reach the fuel lines.
WARNING: Winter Blends
In colder months, stations lower ethanol content to help with cold starts. Always test your fuel or assume E70 in winter. The EIA notes that blends vary by season and region.
Why E30?
For many turbocharged cars (Ecoboost, VW GTI, Subaru WRX), E30 (30% Ethanol) is the "sweet spot". It provides a significant octane bump (often 96-98 equivalent) without maxing out the stock fuel system's volume capacity. Running E30 typically only reduces your MPG by 7% compared to 25%+ on full E85.
Common E85 Mixing Ratios
| Target Blend | Ethanol % | Best For |
|---|---|---|
| E30 | 30% | Stock turbos (Ecoboost, GTI, WRX) |
| E40 | 40% | Upgraded fuel pump |
| E50 | 50% | Upgraded injectors + pump |
| E85 | 51-85% | Full flex-fuel or dedicated build |
Tools You Need
- Ethanol content tester (~$30 on Amazon) – measure actual pump ethanol %
- Our E85 Mixing Calculator – get precise volumes
- Jerry can – for controlled pouring (optional)
Before you start, make sure to check if your car is E85 compatible. Want to know if mixing E85 will save you money?