Make sure the display is compatible with a HD44780 chipset. It can easily be interfaced to PORTB of the PIC16F84 using the 4 bits data bus protocol. In total only 7 datalines are used for the display. That leaves the RB0 free as an interrupt input for the tachosignal. On PORTA a block-wave is generated for testing purposes.

The tachometer source for the PIC16F84 can be found here: TTLCD V0.1.zip

The schematic shows that the display is connected directly to the PIC 16F84. The display is used in 4 bit mode that means that DB0-DB3 need to be connected to ground. The variable resistor is to change the contrast. 

The tacho signal is taken from the bobbin, see the Tachometer for Opel Display page for more details on that. I use PORTA to generate some test signals. You will find different RPM values on RA0-RA4. The test signals are not completely stable but are good enough for testing purposes. The test signals are generated during the waiting routines and are not generated while the PIC is busy doing calculations and so on. Therefore the signal comes in bursts, which results in instability. 

I have not tested this project in a car. And I do not intend to do so either. I have only tested it with the onboard test signals, see photo. However Manuel has build it to be used on his lathe. See: Tacho for a lathe.