While I was driving my car suddenly the left control arm suddenly broke off (the car has had a curb accident on that side before) and the cv-joint de-attached from the transmission and the car was not moving anymore while in gear. But something I noticed is the vehicle speedometer was relative to the RPM; as the car was in gear and increasing the RPM the speedometer was functioning as if the vehicle was in motion. The car in question is a regular FWD with Front Engine Layout.
So does the ECU just use the rpm, gear ratio of the current gear and final drive ratio to calculate vehicle speed? what if the engine wasn't so healthy and was not producing the torque it used to say 10 years ago at the same rpm, is the speedometer now inaccurate because the motor losing its power? Is it calculated differently for different engine/drive setups?