https://www.youtube.com/watch?v=BwN92dBCwog
Demonstration of my arduino nano based tachometer for VW T3/T25/Vanagon with diesel engine. Can be adapted to any car whose alternator has a W output plug. Sorry for low quality video...

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Designed for Arduino Nano and Uno, but can be easily adapted to other models.

Schematic of the acquisition stage and arduino code available for download there --> http://www.allory.net/t3/
Output of the acquisition stage goes to pin 2 (aka INT0).

The screen is a refurbished Nokia 5110 from China that you can find for just a couple of bucks, just as the PC817 optocoupler. 
Screen actual pins assignement:
LCD Chip Select = 8
LCD reset, active low = 7
LCD data/command = 6
LCD SPI Mosi = 5
LCD SPI clk = 4
/!\ Screen and backlight operates 3.3V
Idea for voltage adaptation there: http://www.avdweb.nl/arduino/hardware-interfacing/nokia-5110-lcd.html?