I would like to modify my car with the help of Arduino to display the current gear. I wasn't quite sure where should I post this question as it is a car topic, as well as an electric engineering and mathematics topic.
I want to get the engine rpm and current speed (and any additional information I need) via the OBD2 port of my car. The best solution would be with a small cable adapter so I could simply hide the cable behind the dashboard. The OBD2 for Arduino adapter is just too big, unfortunately the connector in my car is under the steering wheel so it should be as compact as possible as I don't want to accidentally kick it. For the Arduino part I would like to use an ATtiny85 (since it is so compact) and a small display, 7 segment or LED matrix.
The math part is to calculate the current gear from the OBD data. How should I do it? Is it even possible? There is an extension for the Torque app (RacingMeter) that shows the current gear, I am not sure how, but that is where the idea came from.