Here is a collection of some of the Arduino projects I have made. These have automotive applications. Some are just random tinkering. The Arduino UNO is a great platform because the chip is not soldered down. If you have the space for it, then you can plug it into your own boards with ease.
Control a Pioneer radio in your car using the steering wheel buttons
Replacement gauge cluster for the Pontiac Fiero
Air fuel ratio watchdog, somewhat generic, but at home in the Fiero cluster. Also a uses infrared transmitting to a Pioneer CD/MP3 player.
Simple analog read in and convert to true analog output
Read in true analog over SPI bus to the Arduino
The ADC and DAC projects combined into one project
Fake Load Simulation Device For Use In A Car Or Anywhere Really