diff --git a/firmware/controllers/algo/engine_configuration.cpp b/firmware/controllers/algo/engine_configuration.cpp
index a702b00de4..e94d52a6f4 100644
--- a/firmware/controllers/algo/engine_configuration.cpp
+++ b/firmware/controllers/algo/engine_configuration.cpp
@@ -792,6 +792,7 @@ static void setDefaultEngineConfiguration(DECLARE_ENGINE_PARAMETER_SIGNATURE) {
 
 	setLambdaMap(config->lambdaTable, 1.0f);
 	engineConfiguration->stoichRatioPrimary = 14.7f * PACK_MULT_AFR_CFG;
+	engineConfiguration->stoichRatioSecondary = 9.0f * PACK_MULT_AFR_CFG;
 
 	setDefaultVETable(PASS_ENGINE_PARAMETER_SIGNATURE);
 
diff --git a/firmware/controllers/algo/fuel/fuel_computer.cpp b/firmware/controllers/algo/fuel/fuel_computer.cpp
index 327c2f9f0b..b7c69b11fd 100644
--- a/firmware/controllers/algo/fuel/fuel_computer.cpp
+++ b/firmware/controllers/algo/fuel/fuel_computer.cpp
@@ -22,14 +22,33 @@ FuelComputer::FuelComputer(const ValueProvider3D& lambdaTable) : m_lambdaTable(&
 
 float FuelComputer::getStoichiometricRatio() const {
 	// TODO: vary this with ethanol content/configured setting/whatever
-	float rawConfig = (float)CONFIG(stoichRatioPrimary) / PACK_MULT_AFR_CFG;
+	float primary = (float)CONFIG(stoichRatioPrimary) / PACK_MULT_AFR_CFG;
 
 	// Config compatibility: this field may be zero on ECUs with old defaults
-	if (rawConfig < 5) {
-		return 14.7f;
+	if (primary < 5) {
+		// 14.7 = E0 gasoline AFR
+		primary = 14.7f;
 	}
 
-	return rawConfig;
+	// Without an ethanol/flex sensor, return primary configured stoich ratio
+	if (!Sensor::hasSensor(SensorType::FuelEthanolPercent)) {
+		return primary;
+	}
+
+	float secondary = (float)CONFIG(stoichRatioSecondary) / PACK_MULT_AFR_CFG;
+
+	// Config compatibility: this field may be zero on ECUs with old defaults
+	if (secondary < 5) {
+		// 9.0 = E100 ethanol AFR
+		secondary = 9.0f;
+	}
+
+	auto flex = Sensor::get(SensorType::FuelEthanolPercent);
+
+	// TODO: what do do if flex sensor fails?
+
+	// Linear interpolate between primary and secondary stoich ratios
+	return interpolateClamped(0, primary, 100, secondary, flex.Value);
 }
 
 float FuelComputer::getTargetLambda(int rpm, float load) const {
diff --git a/firmware/integration/rusefi_config.txt b/firmware/integration/rusefi_config.txt
index b92e00b685..9e24f8d416 100644
--- a/firmware/integration/rusefi_config.txt
+++ b/firmware/integration/rusefi_config.txt
@@ -1421,11 +1421,12 @@ tChargeMode_e tChargeMode;
 	int16_t idlerpmpid_iTermMin;iTerm min value;"",        1,     0,  -30000,    30000.0,  0
 
 	spi_device_e tle6240spiDevice;
-	uint8_t stoichRatioPrimary;+Stoichiometric ratio for your primary fuel.;":1", {1/@@PACK_MULT_AFR_CFG@@},0, 5, 25.0, 1
+	uint8_t stoichRatioPrimary;+Stoichiometric ratio for your primary fuel. When Flex Fuel is enabled, this value is used when the Flex Fuel sensor indicates E0.;":1", {1/@@PACK_MULT_AFR_CFG@@},0, 5, 25.0, 1
 	int16_t idlerpmpid_iTermMax;iTerm max value;"",        1,     0,  -30000,    30000.0,  0
 
 	spi_device_e mc33972spiDevice;
-	uint8_t[3] unusedSpiPadding8;;"units", 1, 0, -20, 100, 0
+	uint8_t stoichRatioSecondary;+Stoichiometric ratio for your secondary fuel. This value is used when the Flex Fuel sensor indicates E100.;":1", {1/@@PACK_MULT_AFR_CFG@@},0, 5, 25.0, 1
+	uint8_t[2] unusedSpiPadding8;;"units", 1, 0, -20, 100, 0
 
 
 	float etbIdleThrottleRange; ETB idle authority; "%", 1, 0, 0, 15, 0
diff --git a/firmware/tunerstudio/rusefi.input b/firmware/tunerstudio/rusefi.input
index 95e88c1fd7..ab921e2479 100644
--- a/firmware/tunerstudio/rusefi.input
+++ b/firmware/tunerstudio/rusefi.input
@@ -1807,7 +1807,8 @@ cmd_set_engine_type_default					= "@@TS_IO_TEST_COMMAND_char@@\x00\x31\x00\x00"
 		field = "Injector reference pressure", fuelReferencePressure, { isInjectionEnabled && injectorCompensationMode != 0 }
 
 	dialog = fuelParams, "Fuel characteristics", yAxis
-		field = "Stoichiometric ratio",		stoichRatioPrimary, {isInjectionEnabled == 1}
+		field = "Stoichiometric ratio",			stoichRatioPrimary,		{isInjectionEnabled == 1}
+		field = "E100 stoichiometric ratio", 	stoichRatioSecondary,	{isInjectionEnabled == 1 && flexSensorPin != 0 }
 
 	dialog = injectorOutputSettings, "Injector Outputs",	yAxis
 		field = "Use only first half of pins for batch mode"
diff --git a/unit_tests/tests/ignition_injection/test_fuel_computer.cpp b/unit_tests/tests/ignition_injection/test_fuel_computer.cpp
index 3ed1da8855..dabaef5a87 100644
--- a/unit_tests/tests/ignition_injection/test_fuel_computer.cpp
+++ b/unit_tests/tests/ignition_injection/test_fuel_computer.cpp
@@ -42,3 +42,39 @@ TEST(FuelComputer, LambdaLookup) {
 
 	EXPECT_FLOAT_EQ(dut.getTargetLambda(1500, 0.7f), 0.85f);
 }
+
+TEST(FuelComputer, FlexFuel) {
+	WITH_ENGINE_TEST_HELPER(TEST_ENGINE);
+
+	MockVp3d lambdaTable;
+	FuelComputer dut(lambdaTable);
+	INJECT_ENGINE_REFERENCE(&dut);
+
+	// easier values for testing
+	engineConfiguration->stoichRatioPrimary = 150;
+	engineConfiguration->stoichRatioSecondary = 100;
+
+	// No sensor -> returns primary
+	Sensor::resetMockValue(SensorType::FuelEthanolPercent);
+	EXPECT_FLOAT_EQ(15.0f, dut.getStoichiometricRatio());
+
+	// E0 -> primary afr
+	Sensor::setMockValue(SensorType::FuelEthanolPercent, 0);
+	EXPECT_FLOAT_EQ(15.0f, dut.getStoichiometricRatio());
+
+	// E50 -> half way between
+	Sensor::setMockValue(SensorType::FuelEthanolPercent, 50);
+	EXPECT_FLOAT_EQ(12.5f, dut.getStoichiometricRatio());
+
+	// E100 -> secondary afr
+	Sensor::setMockValue(SensorType::FuelEthanolPercent, 100);
+	EXPECT_FLOAT_EQ(10.0f, dut.getStoichiometricRatio());
+
+	// E(-10) -> clamp to primary
+	Sensor::setMockValue(SensorType::FuelEthanolPercent, -10);
+	EXPECT_FLOAT_EQ(15.0f, dut.getStoichiometricRatio());
+
+	// E110 -> clamp to secondary
+	Sensor::setMockValue(SensorType::FuelEthanolPercent, 110);
+	EXPECT_FLOAT_EQ(10.0f, dut.getStoichiometricRatio());
+}