Self contained, digital tach with built-in memory. Small 3 1/8" x 3 1/8" size. Tel Tac II recalls and displays the highest overall RPM for 30 minutes. Tel Tac Oval Track Pro shows the highest overall RPM, the four lowest RPMs and the four highest RPMs from two laps. Powered by common 9 volt battery. Works on 4-6-8 mags or distributors. Extremely rugged construction, Built to work in almost any racing enviroment.