This is a OLED dashboard module for the Volvo 240 that provides speedometer, tachometer, and battery voltage.
The PCB layout is in KiCAD. A BOM used to purchase board components at Digikey can be found in the PCB Layout directory as well.
The back of the dashboard cluster has an open collector signal for the speedometer pulses (reverse engineered with help from http://cleanflametrap.com/speedo.html). This already has a 12V pullup on it, so all you need to do is connect a lead to the output.
The tachometer signal can be connected to the red and white wire present behind the gauge cluster (not connected in 240s without a tach from the factory).
12V Power and ground can be spliced into the 3-pin power cable for the gauge cluster.
The Arduino code is compiled for the ATtiny84. You will need the ATTinyCore board library.