[3fe9a03ccd14ca5]

Really cool hack and fun idea! Some ideas if your only goal is to get a speedometer. Firstly, ditch the pi and use something with much less power usage, such as an ESP32 or Arduino or STM32 whatever. The Pi uses a lot of power. Then use a GPS IC such as the Neo 6M[1], which is cheaper and less power usage than a USB GPS dongle.

The point of having a low-power SoC is that it can stay on while the car is powered off, taking a GPS sample every 10 minutes or so, so that when the car powers on it can very quickly get a lock. The Neo 6M also keeps a memory of the last location which helps it get a lock quickly.

https://create.arduino.cc/projecthub/ruchir1674/how-to-inter...

[oh_sigh]

Even relying on GPS is overdoing it. Paint a white stripe on a tire using a permanent marker, and then mount an IR emitter and phototransistor in the wheel well. Knowing the diameter of the tire and the time between revolutions lets you easily figure out speed.

[3fe9a03ccd14ca5]

Great idea! But I think this is harder than GPS because the chip simply spits out speed and direction.