Sigh. Speed is distance moved per unit of time. The distance moved on a trainer, assuming you're doing it right, is zero. So your speed is zero. There are other metrics you can use to gauge your workout...power, PE, etc. But "speed" isn't the most reliable, IMO.
I ride my trainer almost every single day, and have NO idea what my cpu says, as I keep it covered (actually, the battery in it died about 18 months ago, so I couldn't see what it said even if I wanted to). I log my trainer workouts based on time and effort.
As far as your tire PSI...that depends on your tires and your trainer.
Agreed, I use HR cuz that is what I have. I like to log milage and time and was checking around. For spin classes I just use 18mph. The instructor says 20 but I like to be conservative. And, yes everyone is different I know.