Hit your target.
Mix it right.
Use this calculator to quickly work out how much ethanol and pump fuel to mix to hit your target blend. Enter your tank size, current fuel level and desired ethanol content — the calculator shows exactly how much to add.
Quick targets
Add to tank
Resulting mix
50% ethanol
≈ E50
Math assumes you fill to full and that ethanol % values are accurate. Real E85 varies by pump and season — verify with a flex sensor or test kit if precision matters.
Background
What is ethanol,
and why use it?
Ethanol is an alcohol-based fuel, commonly found in fuels like E10, E85 and higher blends. Compared to standard pump fuel, ethanol has a higher octane rating, burns cooler, and allows the engine to run more efficiently under load.
When tuned correctly, the gains are real:
- · More power — higher octane allows for more aggressive tuning
- · Lower temperatures — helps reduce heat under load
- · Improved consistency — especially in performance and track use
It’s one of the most effective ways to increase performance on modern turbocharged engines.
Why precision matters
Get the mix right —
every time.
Getting the mix right is critical. Too little ethanol and you don’t gain the benefits. Too much ethanol and the car may not run correctly without further tuning support.
This calculator removes the guesswork and helps you:
- · Build accurate blends (E30, E40, E50 and beyond)
- · Stay consistent every time you fill up
- · Match your fuel to your tune
Simple, fast and essential if you’re running — or planning to run — ethanol.
How to use it
Why it asks
six questions
The calculator is most important when you’re topping up on E30 (or any blend) and there’s already mixed fuel in the tank. You can’t use the same ratio every time — get it wrong and the car runs too lean.
- 01
Current ethanol mix
What's already in the tank. If it's all fresh pump fuel, that's whatever your local station's ethanol percentage is. If you topped up on a flex blend last time, enter that. A flex sensor or ethanol test kit gives the exact number.
- 02
Current fuel level
Roughly how full the tank is — your gauge is fine, or set up a 'Fuel Level Input' gauge on your Bootmod3 / MHD / accessport for a live percentage. 25% means a quarter of your tank size is currently sloshing around in there.
- 03
Pump gas ethanol %
Always account for the ethanol that's already in your pump fuel. UK pumps sell E10 (10% ethanol) as standard since 2021, with E5 (5%) often available as Super/Premium. In the US, regular is typically E10 — only midwest pumps explicitly marked ethanol-free are 0%. Enter that — even 10% goes a long way.
- 04
E85 actual ethanol %
Real-world E85 isn't always 85% ethanol. Seasonal blends in the UK / EU / US sit between E70 and E83. Test if you can. If unsure, 75% is a safe assumption.
- 05
Tank size
Capacity in litres or US gallons (toggle above the inputs). Most BMW chassis run 50–80 L (13–21 US gal).
- 06
Target ethanol mix
What you want to end up at. Quick-pick E30, E50 or E85, or type any number. Higher ethanol = more cooling, more knock margin, more power on a flex map — but also more fuel demand on stock injectors and pumps.
Why precision matters
Lean = knock = damaged engine
A flex-fuel map expects a known ethanol %. Run too lean (less ethanol than the map assumes) and combustion temps spike, knock margin disappears, and worst case you melt a piston. Get the mix right.
Hardware floor
Stock fuel system tops out around E50
On B58 / S58, the stock LPFP and HPFP comfortably support E30–E50. Pushing past that for sustained use needs a pump upgrade — Samuel will flag this when you brief your build.
Map first, mix second
The map locks the target
Stage 2 Flex Fuel and Stage 2+ each assume a target ethanol %. Confirm yours with Samuel before you mix — the calculator gets the volumes right; the map gets the timing right.
Got your blend?
Build the matching tune.
Samuel calibrates flex-fuel maps to your exact target. Brief him with the % you’re running and the hardware behind it.