This gauge is a drop in replacement for the original Pontiac Fiero coolant temp and fuel level gauge without the need for any cutting or splicing into anything on the car. The drive to build this device is what lead me to purchase my first Arduino in 2012 (and the code looks like someone's first project). Everything was built and tested on the UNO and then the chip was upgraded to the high temp version and moved to its own PCB. The full documentation can be found at SpeedLimit88.com.
There are two digits for the fuel level on the bottom half and three digits for the coolant on the top half. The three digits on the top will start off as the coolant display and can be changed to show battery voltage (and other sensor values as chosen). The fuel level reads from 99 (full) to 00 (empty). When it is 10 or lower, a yellow LED to the left of the number lights up. The coolant gauge reads from 0 degrees Fahrenheit up to well past 260F. The display has a light sensor also to the left of the fuel level display. It will adjust the brightness of the LED’s automatically without the use of the parking light dimmer switch, but when the dimmer switch is turned on, it will dim the display with the dimmer switch.
All files and BOM: http://www.speedlimit88.com/arduino/multigauge/
There is quite a bit I would do differently if I did it all over again, but the original is still in my car and still working.