Fuel & Tuning
Air-Fuel Ratio ↔ Lambda Converter
Convert air-fuel ratio to lambda and back for gasoline, E10, E85, E100, methanol, diesel, or a custom stoich — and see the equivalent AFR on every fuel at the same lambda.
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How to Use This Tool
- Choose your fuel or enter a custom stoichiometric ratio (for example from an ethanol content sensor).
- Type an AFR or a lambda value — the other updates instantly.
- Use the fuel comparison table to translate a target between fuels at the same lambda, such as a gasoline 12.5 target on E85.
- Check the typical target table as a starting point, then confirm final mixtures with your tuner and a wideband.
The Formula & Physics Behind the Math
Lambda (λ) is the actual air-fuel ratio divided by the stoichiometric ratio for that fuel: λ = AFR ÷ stoich, so λ 1.00 is chemically correct combustion, below 1 is rich, and above 1 is lean. Because the definition is normalized, the same lambda means the same relative mixture on any fuel — AFR = λ × stoich. Stoichiometric ratios used here are approximately 14.7 for gasoline, 14.1 for E10, 9.8 for E85, 9.0 for pure ethanol, 6.4 for methanol, and 14.5 for diesel; real pump fuel varies with blend and additives. Many wideband gauges display gasoline-scale AFR (λ × 14.7) regardless of fuel, which is why lambda is the safer number to tune in.
Frequently Asked Questions
What is lambda in tuning?
Lambda is the measured air-fuel ratio divided by the fuel's stoichiometric ratio. Lambda 1.0 is stoichiometric, lower numbers are rich, and higher numbers are lean. Tuners prefer lambda because it means the same thing whether the car runs gasoline, E85, or methanol.
What AFR is 12.5 on gasoline when running E85?
12.5:1 on gasoline is about λ 0.85. At the same lambda, E85 with a 9.8 stoich runs about 8.3:1. If your wideband is set to display gasoline AFR, it will still read 12.5 — the gauge is just multiplying lambda by 14.7.
Why does my wideband show 14.7 on E85?
Most wideband controllers measure lambda and then display it on a gasoline scale by multiplying by 14.7. On E85 that 14.7 reading is really λ 1.00, or about 9.8:1 actual AFR. Switch the gauge to lambda or set the correct stoich to avoid confusion.
What lambda should I run at wide-open throttle?
Typical starting points are around λ 0.85–0.90 for naturally aspirated gasoline engines and λ 0.75–0.82 for boosted gasoline engines, with ethanol often run a touch leaner in lambda terms thanks to its cooling. These are general references only — the right target depends on the engine, fuel, and timing, so work with a tuner.