An OBDII gauge. Wanted to make one myself for the learning experience, and to customize it to show what I'd like.
Nothing crazy, but its easy to swap in your own metrics and customize. The screen is set to power off automatically if no OBD data is recieved, so it shouldn't drain your battery if left plugged in.
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Building this is pretty straightforward! Just follow the docs for the display and OBD adapter on how to wire it up. I'm using the default pins.
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You'll want to install these via the Arduino library manager:
Adafruit_GFX
Adafruit ST7735 and ST7789
You will also need the OBD2UART library. If you're on a Mac you can grab this by running the helper.sh script.
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The general idea is to have it grab the PIDs you want to display. You can see the available PIDs here. There's also a wiki article that lists them, in case any are missing.
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Use the Arduino IDE as normal. The only caveat is you may need to disconnect the UART RX pin before uploading the image, else you'll get errors. You can just unplug the 4 pin connector that goes into the OBDII adapter to work around this.