It seems as though the motor controller is a little on the high side during the calibration process.
To fix this, adjust the pot whilst in calibration mode. You should be able to press the speed up button and the treadmill should start moving, showing you a percentage number on the incline window.
First, increase the speed until that number shows 85%. The speed window should show you what speed the treadmill is actually moving at. At this point, the console may or may not be beeping.
If at 85%, the speed shows 12mph, or very close to it (and there are no beeps from
the board or console while moving), then the treadmill is properly calibrated, and
your speed perception or expectation is just not what the treadmill offers.
If at 85%, the speed indicates it is moving lots faster than 12mph, you hear a fast beep from the console, or even feel like it may overheat, then you will now need to adjust the speed pot by doing the following:
While at 85%, and the belt still moving, turn the pot counterclockwise until the speed indicates as close to 12 mph as possible, or if the console was beeping fast, do it until the beeping stops.
Be careful to turn the pot slowly, as it is very sensitive, and one small turn will lower or increase the speed by a lot.
When the beeping stops, or when you are running the treadmill at 85% at the calibration window (and the speed reads close to 12mph), the treadmill will be calibrated. Now .5mph should now be .5, and 12mph should now be 12mph.
Pull the safety key off and put it back on to reset the calibration window to the normal starting point.
Let me know if that works for you?