This is from an old post in the power thread.

This was recovery ride on a trainer. Power is yellow, speed is blue.
The power output and settings (same bicycle gear, no adjustments in the trainer) were constant, yet the speed dropped during the first 25-30 minutes. I asked if somebody knew why.
A proposed explanation was that thru your workout, the trainer heats up and the materials expand, causing the resistance to increase, and the speed (recorded with a sensor in the rear wheel, in contact with the trainer) to drop.

You might want to keep this in mind if you want to use speed on the trainer as a surrogate for power.