Flex fuel sensors measure ethanol content. They can be used in the FPR return line. But for improved accuracy and faster response times, the sensor should be plumbed upstream of the fuel injectors in the feed line. In this case, expect to see up to a ~10psi pressure drop if multiple pumps are used with a high-powered engine. 

As shown above, the Radium Engineering adapter splits the fuel flow into an auxiliary passage parallel to the sensor. Because the flex fuel sensor is directly inline with the fuel flow path, it will read the incoming ethanol content at high and low engine demands. Traditionally this additional flow path could be accomplished with hose and y-adapter fittings. However, the Radium Split-Flow Flex Fuel Sensor Adapter is less complicated, considerably compact, and provides a mounting solution for the popular GM 13577429 flex fuel sensor.