I've been working on a project for a few weeks now, so it's time to introduce it !
My project is a LCD dash for Honda cars. it connects to the car's Engine Control Unit and fetches data to display it on a KS0108 based LCD.
I've chosen to use the Mega to achieve this because of its multiple hardware serial ports.
The arduino connects to the ECU via a ttl serial connection, so I could have used the regular arduino but I wanted to keep the USB serial port for monitoring or future features.
I have also used an algorithm for the temp sensor which could be useful for you.
I plan to rely on the ECU data mostly yes. I will expand the compatibility to standalone ECUs for race cars as soon as I get my hands on the protocols.
Also, I might switch to a bigger LCD, with T6963 controllers, which has some interesting hardware drawing functions as well.