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Air to Fuel ratio (AFR)


Air to Fuel Ratio (AFR)

Occasionally we are asked if UltraGauge supports real-time AFR. We have in the past not supported it.  Note that it is supported by the UltraGauge MX and Blue as a user programmable parameter, but not recommended.  Please read on to understand why.

Real-time AFR can be determined on those vehicles which support wideband O2 sensors.  Wideband O2 sensors are less common, but very much superior to the older narrow band O2 sensors. Narrow Band O2 sensors have a very narrow range of useable operation around the Stoichiometric ratio. The Stoichiometric ratio is the point at which there is just the exact amount of Oxygen to burn the available fuel.  The narrow band sensors essentially can only tell the ECM that the mixture is lean or that it’s rich, but not the degree. In fact if you monitor the O2 sensor output, it constantly switches from lean to rich, and rich to lean, as the ECM attempts to keep the Air to Fuel mixture at the Stoichiometric point.

Wideband O2 sensors have a much broader linear range of operation and if monitored generally provide a relatively constant output corresponding to the amount of oxygen in the exhaust.  As a result, wideband O2 sensors can be used to provide real time AFR. The ECM monitors the wideband O2 sensor and outputs the ratio Lambda. 

Lambda = Actual AFR / Stoichiometric AFR.

When the AFR is ideal, Lambda is 1. When the mixture is Rich, actual AFR is reduced and Lambda is less than 1. 

If the Stoichiometric AFR is known for the fuel in use, then the Actual AFR can be determined

Actual AFR = Lambda * Stoichiometric AFR = (Actual AFR / Stoichiometric AFR) * Stoichiometric AFR

But here in lies the problem.  The Stoichiometric AFR is never known because the makeup of the fuel that comes from the pump is not known.  For example, this table provides the Stoichiometric AFR for various ideal fuels



Stoichiometric AFR

Pure Gasoline


10% Ethanol Gas


15% Ethanol Gas




Pure Ethanol





The problem is that pure gas is never pure, and a 10% blend is rarely 10%. That's why the pumps reads: "May contain 10%...". But in reality, it could be 1% or 15%, or any percentage in between.


Without knowing the Stoichiometric AFR for the fuel in your tank, there is no way to use wideband O2 sensor and lambda to determine exact value of AFR. Most AFR meters simply assume pure gasoline and use a value of 14.7:1.  However, the O2 sensor cares little that you are using pure gas or pure Ethanol.  For both it will report a Lamda of 1.

So let's say you have E85 in the tank. What will your AFR meter read? It will read 14.7:1, because Lambda is 1. But we know the AFR should be around 9.75:1. This is why reporting AFR can be so misleading and absolutely wrong.

The far better parameter to monitor is Lambda, as Lambda is independent of the fuel used.   As long as Lambda is very near or equal to 1, you know your mixture is correct (Stoichiometric).   If for performance reasons, you still wish to monitor AFR, because you wish to run rich, Lambda is still the better parameter to monitor as AFR will be distorted by the Stoichiometric AFR assumed.   Using the MX or Blue, you can program  any Stoichiometric AFR you wish, but it is still best to simply use Lambda.

* Diesel engines do not run at the Stoichometric point and the actual AFR varies from 14 to as much as 70 (lambda >>1).   Diesel MPG accuracy

Related articles MPG Accuracy
Diesel MPG Accuracy & Calibration
Fuel Sender detection
Article details
Article ID: 29
Category: Knowledgebase

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