Vehicle Speed From Engine And Generator Speed
WARNING: This page is about a different car, the 2010 Mercury Mariner and 2010 Ford Escape. However, it is still accessible from the selected car via links, so may be relevant.
The TCM calculates generator speed from the generator shaft speed sensor and sends this information to the PCM. The PCM combines input information from the generator speed and engine speed along with tire size and gear ratio to calculate a vehicle speed.
The PCM strategy then cross checks all the inputs to determine if they agree with one another.