Both the Arduino and the PC side software are now updated to process the speed and RPM signals coming from Live For Speed.
What's interesting, is that the Arduino (ATMega1280) being single-theaded, I had to write all the code in one function. So, there is only one thread which reads data from the serial port, and as soon as a complete package is received, it updates the control lights and speed / RPM signals.
The speedometer and rev counter are fed with clock signals, whose frequencies vary according to the speed / RPM to be displayed on the dial (the actual speed and RPM sensors on the real car read their values from the rotating wheels/engine crankshaft, thus sending clock-like signals to the instrument cluster). These signals are generated by means of two dedicated timers, also implemented in the same single thread.
Here's a video of the real thing working in LFS.