Several possibilities, based on your responses, 1) was your pickup a V8, V6, I6, or I4 and does the tach have a strap or switch option to correctly indicate RPM for a VTwin configuration? 2) Do you have a mounting area to accomodate the 2 1/2 Tach with appropriate bracket?, 3) There are some diagnostic modes that may not register, or could kick out codes which you could live with?