Originally Posted by
pdedes
trainer resistance doesn't necessarily correspond with outside resistance. a watt, however, equals a watt
This.
You need to measure your effort on a trainer by power output, not by speed. Depending on the trainer, your size, how aero you are on the bike, wind speed, direction, etc. 20 mph on a trainer can be radically different than 20 mph on even a flat road.
Ideally you want a power meter to measure your effort; next best is heart rate, but its subject to drift working indoors as you get hot. Last is perceived effort.
The only relevance of speed on a trainer is if you have a known power curve for your trainer, and you can extrapolate power from speed using the known curve.