Target & tank
Defaults to E40. Any target works.
What the tank holds when full
In the tank now
From your flex-fuel sensor or your last blend
Fuels to add
US E85 really contains 51–83% ethanol, depending on the season. Test it if you can.
How to fill
The calculator picks fuel 2 and the amount for you.
Ratio table
Ethanol content of an empty-tank blend. Rows are the mix ratio (low-ethanol : high-ethanol, by volume). Columns are the tested content of the high-ethanol fuel.
- Both optimal
- Column optimal
- Row optimal
- Your tested content
- Grey text = below target
Column optimal (green): for that tested content, the ratio with the most low-ethanol fuel that still meets the target. It uses the least E85.
Row optimal (blue outline): for that ratio, the lowest high-ethanol content that still meets the target.
Both optimal (dark green): optimal for its row and its column at the same time.
Optimal cells are picked from unrounded values. Displayed blends always round down, so a cell below target never shows as the target. The second header line is the exact ratio for that column.
Prices at this station
Leave a fuel blank if the station doesn't sell it.
Cheapest mix
Price per gallon of what you add, averaged over both fuels. Every option ends at the same blend.
Formula
Efinal = (ER·VR + EL·VL + EH·VH) ÷ (VR + VL + VH)
R is the fuel already in the tank, L the low-ethanol fuel, H the high-ethanol fuel and T the target. E is ethanol percent by volume (E40 is 40) and V is volume.
Both fuels, adding A in total:
VH = [ET·(VR + A) − ER·VR − EL·A] ÷ (EH − EL), VL = A − VH
Fuel 2 after pumping fuel 1:
V2 = [(ET − ER)·VR + (ET − E1)·V1] ÷ (E2 − ET)
Empty-tank ratio:
VL : VH = (EH − ET) : (ET − EL)
- The math runs at full precision. Results show 2 decimals and always round toward meeting the target: high-ethanol amounts round up, low-ethanol amounts round down, and blend percentages round down.
- When the E85 content is uncertain, the plan uses the lowest value, so even the worst case meets the target.
- Volumes simply add up. The small volume change when ethanol and gasoline mix is ignored.
- Results are a guide only. Trust your ethanol sensor or a test kit for the real content.
Add to home screen
In Safari, tap Safari's Share button, then Add to Home Screen. It opens full screen like an app.
Open it once from the home screen while you have signal. After that it works with no connection, and updates arrive by themselves when you're back online.
To send it to someone, tap Share at the top of this page.
Settings and prices stay on your device.
The public copy at dosmiao.github.io/e40-blend can be added to your iPhone home screen and works offline. Open it in Safari, tap Share, then Add to Home Screen.
Send that link to anyone who wants it. Settings and prices stay on each person's own device.
Reset data
Clears the settings and station prices saved on this device, like clearing this site's cookies, and puts every field back to its default.