The XFI Driveshaft Speed Sensor Kit from FAST accurately measures driveshaft speed to provide better data for the traction control unit. Two magnets are fitted to the collar around the pinion yoke, which attaches to the driveshaft. When the magnet passes the pickup, it sends a signal to the XFI unit. The XFI uses that signal to calculate driveshaft RPM and also driveshaft acceleration. As a result, the XFI Driveshaft Speed Sensor Kit provides a much more accurate data reading than attempting to determine driveshaft speed solely through engine RPM. Perfect for drag race applications, this unit also provides a MPH reading, and, if the user enters tire size, miles per gallon.