Interesting idea!

I think the first challenge would be: how can the RPi display something on the Infotainment System's display?

First challenge would be:
How do you turn ON the Infotainment System when one opens the door? I assume the Infotainment System is off when you open the door of your car? (mine goes on after I either insert the ignition key or start the engine)

Next challenge is:
How would you connect it to the RPi, sot the RPi can display something on the screen?
I know some Infotainment Systems have an analog video IN connector, some maybe even a HDMI input, but for that the Infotainment System may need to be set to AUX or something like that?

All in all, I do not think this will be a straight forward thing to do. Unfortunately, since I have some ideas of my own when it comes to that 😁 (eg. use a RPi to make Apple Carplay or Android Car possible)

Posted by: @hans

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Interesting idea!

I think the first challenge would be: how can the RPi display something on the Infotainment System's display?

First challenge would be:
How do you turn ON the Infotainment System when one opens the door? I assume the Infotainment System is off when you open the door of your car? (mine goes on after I either insert the ignition key or start the engine)

Next challenge is:
How would you connect it to the RPi, sot the RPi can display something on the screen?
I know some Infotainment Systems have an analog video IN connector, some maybe even a HDMI input, but for that the Infotainment System may need to be set to AUX or something like that?

All in all, I do not think this will be a straight forward thing to do. Unfortunately, since I have some ideas of my own when it comes to that 😁 (eg. use a RPi to make Apple Carplay or Android Car possible)

Hi, I'll explain a bit more of what i was thinking. 

- The raspberry pi will use its own os system Raspberry os, which the raspberry pi will be connected to the official raspberry pi 7inch touch screen. Which I will also connect via hdmi to a aftermarket double din head unit, as a secondary display.

- I want the raspberry pi to turn on fully and off when I turn the car on with the ignition. However display a logo and the milage when a door opens. Then turn off or sleep when the door closes. Thus does not boot up the raspberry pi. It only boots up when the ignition is on. 

Thanks

I suppose turning on the RPi is easy - make sure the RPi power is connected to a power connection in your car that turns on when the car door is unlocked, door opened, or car started.

Turning off with a delay may not be needed, depending on the OS you use for your RPi.

I could be wrong, but I think embedded Linux is OK with instant power off, and should be available for RPi.
Example: LibreElec runs on an embedded Linux version - granted, LibreElec is intended for a media player [KODI], but it shows that such a distro should does exist - there are a few articles out there that should get you started, like this article.
* This to prevent filesystem corruption when the RPi is powered off.

You could of course add a small battery, for example a cheap power bank, to buffer the power when power is lost - this would make things a little more complicated as one needs to detect power loss and power being turned on again.

So from a on/off/hardware perspective this should work.

Splash image

Now, modifying the boot image can be done like so (source - I have not tested this):

1. In your favorite image editor create a splash screen and save it as splash.png.
   Choose a 1920 x 1080 resolution as it scales well with 16:9 resolutions.
2. Copy the image to a USB flash drive and insert the drive into the Raspberry Pi.
3. Copy splash.png from the USB flash drive to your home directory /home/pi/

4. Open a Terminal on your Piu and change directory to the location of the original splash screen for your Pi.

   $ cd /usr/share/plymouth/themes/pix/

5. Optional: Backup the original splash.png file to a file called splash.png.bk

   $ sudo mv splash.png splash.png.bak

6. Copy your splash.png to the current directory. This will copy the freshly created splash image ready for use.

   $ sudo cp /home/pi/splash.png ./

7. Reboot your Raspberry Pi.
   Once your Raspberry Pi reboots you will see the familiar rainbow screen, and then a few seconds later you will see your custom splash screen. 

Disable boot rainbow

The rainbow at boot can be disabled by adding this to the config.txt file:

disable_splash=1

This should not affect the splash image.

Reading Speedometer

For this you can either use an ODB2 device (cheap and probably easiest) or a GPS module. The ODB2 module provides much more car related data, like RPM etc. Here is a project I found at Instructables that hooks up a ODB2 dongle to your RPi (GitHub backup of that same article).

Hope this helps to get started.