LCD

So for display purposes I have one main LCD in the car 

It is a Matrix Orbital screen that can connect via serial or I2C.  I have chosen I2C.  I2C allows many devices to be hooked up to the same two pins using addresses, similar to how USB works.

As stated above, this will be my main screen for now.  It will show engine information, date/time, speed from GPS unit (which can be seen in the lower left of the picture), iPod song information and whatever else I find necessary.

I also am going to have a small OLED screen from seeedstudio.  It will be placed in the engine compartment and show vital engine stats, such as timing and engine load.  This will be helpful to make sure that what the computer thinks for timing is the same as what it actually is.  Also it will just look cool.

It also speaks via I2C, so no additional pins are required on the mega, just another address.

For my Matrix Orbital, I have a library I found somewhere online and modified for my own use.

LCDI2C library download