AFR Lambda Calculator

Lambda is the universal language

Lambda is your actual air-fuel ratio divided by the stoichiometric ratio of whatever is burning. At λ 1.00 the mixture is chemically complete, below 1.00 it is rich, above it is lean. Every fuel has its own stoich number - 14.7 for gasoline, 9.8 for nominal E85, 6.45 for methanol - but λ 0.850 describes the same mixture strength in all of them: 15% less air than the fuel could burn, which works out to about 18% extra fuel. That is why a tune moved from gasoline to E85 keeps its lambda targets untouched while every AFR number on the sheet changes.

Your wideband probably shows lambda in disguise

A wideband oxygen sensor does not measure air-fuel ratio. It measures the oxygen left in the exhaust, which maps to lambda, and the gauge converts that to an "AFR" display by multiplying by 14.7 - the gasoline constant - no matter what is in the tank. On E85 a display of 12.5 does not mean 12.5 parts of air per part of fuel; it means λ 0.850, a true AFR of about 8.3. The number is still useful, only the label is wrong. Set the gauge to lambda and the confusion disappears, which is the single most common fix in this corner of tuning.

Pump E85 is rarely 85% ethanol

The US pump E85 specification (ASTM D5798) allows the ethanol content to swing seasonally between roughly 51% and 83%, which moves the stoich ratio from about 11.7 down to 9.9. A tune built for one drum of summer fuel can be meaningfully off on a winter fill. A flex fuel sensor reads the real content; the custom ethanol blend option above turns that reading into a stoich value, computed as a mass-weighted blend of gasoline (14.7, 0.74 kg/L) and ethanol (9.0, 0.789 kg/L). The same blend math reproduces the published E10 and E85 values exactly, so the presets and the custom option can never disagree.

Where these numbers come from

The stoich table is nominal, and honest tools say so. Pure gasoline is 14.7 but most US pump gas is E10, which sits near 14.1. Methanol computes to 6.45 by combustion chemistry, with published values running 6.4 to 6.5. Diesel is published at 14.5 to 14.6 and runs lean by design, so rich spark-ignition targets do not transfer to it. None of this spread touches lambda itself: the sensor measures lambda directly, and the stoich constant only enters when you insist on an AFR number. Fueling hardware sized from these targets lives next door - the injector size calculator turns a power target into injector flow, and the boost to horsepower calculator estimates what the turbo is asking the fuel system to feed.