This is Superlift's new TruSpeed Speed Sensor Calibrator. It only measures about 3x6 inches and has a four-wire hookup. The round thing is a key used to calibrate the unit. The key is inserted into a hole in the case, which has an adjustment screw behind it. The screw is rotated a certain number of turns, depending upon your tire diameter and gear ratio. A chart, supplied with the TruSpeed, tells you how many turns to make. Once the turns are made, your speedometer reads dead-on correct, no matter what size tires or gears you swap to.