Air/Fuel Ratio & Lambda Calculator
Lambda is fuel-independent, AFR is not. Convert either way and see the equivalent AFR on every other fuel.
| Air/fuel ratio | 12.50 | : 1 |
| Lambda | 0.850 | λ |
| Stoichiometric for Gasoline (E0) | 14.700 | : 1 |
| same lambda on other fuels | ||
| Pump gas (E10) | 11.97 | : 1 |
| E85 | 8.30 | : 1 |
| Ethanol (E100) | 7.65 | : 1 |
| Methanol (M100) | 5.44 | : 1 |
| Diesel | 12.33 | : 1 |
| Propane / LPG | 13.35 | : 1 |
| Race gas (leaded) | 12.33 | : 1 |
- lambda = actual AFR ÷ stoichiometric AFR
- AFR = lambda × stoichiometric AFR
- Lambda 1.00 is chemically complete combustion on any fuel; a wideband reading in lambda transfers between fuels, an AFR number does not.
When you need it
When tuning, and any time you change fuel.
Lambda travels between fuels, AFR does not
Lambda 1.00 is chemically complete combustion on any fuel. 12.5:1 on gasoline and 8.3:1 on E85 are the same mixture strength — both lambda 0.85 — even though the numbers look nothing alike.
If you tune in AFR and then switch fuels, every number you have memorised becomes wrong, and the direction of the error is not obvious. Set your wideband to display lambda and the problem disappears. This is the single most useful habit to adopt if you run more than one fuel.
Rough targets
Around 0.85 to 0.88 lambda at wide open throttle naturally aspirated. Richer under boost, commonly 0.75 to 0.82, because the extra fuel is doing cooling work as much as combustion work. Stoichiometric at cruise for a closed-loop street engine.
These are starting points for a first pull, not answers. The right mixture depends on the chamber, the fuel, the compression and what exhaust gas temperatures and knock detection are telling you.