I reworked most of the wiring over the past few weeks. I had some problems with the display and decided to make some modifications to the system. The output from the RPM sensor incorporates two transistors in a push-pull formation. It is capable of much higher current.
A capacitor on the display is charged and discharged from the sensor's push-pull output. A voltage comparator converts this into a nice 5 volt square wave for the Microcontroller.