I could use a GPS module to have the car speed, but I think there are two disadvantages. First, we loose it on tunnels, and second I'd like to show what I'm reading on my dashboard.
My first idea was to use a bluetooth OBD plug, and then connect to it with an ESP32. But I did not succeed because the OBD dongle I have uses classic bluetooth and not BLE. In addition I did not found a 12v power line on the trunk that is switching off once the ignition key removed. I don't want to drain flat the car battery.
So I re-purposed the cheap OBDII reader. Luckily it uses an ELM327 clone and the bluetooth module can be de-soldered. That saves me the code of the raw CAN-bus handling, the ELM327 uses simple serial AT style commands. And it saves me the design of the CAN-bus circuitry.