Old 80s cars with an ECU (engine control unit) don't have the luxury of an ODB interface, but often the signal levels are a neat TTL 0-5 volts from the sensors. Engine speed may be a series of pulses, but a frequency to voltage chip will get a nice voltage for you to measure.
This device uses a KS0108 graphical display driven by a nice I2C interface to display the voltages read by a 16F873 PIC on 4 of its A/D pins. The display is just a horizontal bar with a label for the engine parameter being measured - boost (forced induction engine), oxygen (lambda sensor), engine speed and fuelling level.
The strange item on the right is the steering column stereo control from a scrapped Renault Laguna. This is wired as a 3 x 3 matrix with only 7 switches implemented. Two of these switches, [Vol +] and [Vol -], are used here to cycle up and down through the 4 engine parameters being measured so the driver can change this from the steering column.