I find that if I have a long ride, century or something, then take a rest day or two, I'll just feel GREAT and actually will do better.

As to flat-land speed, I never thought to check it like that. If the road is flat and I'm not trying to keep up with anyone, I'm trying to ride at a pace I can sustain, so getting all exhausted just to watch a little number go up hasn't been my thing.

That being said, I also find it very hard to find a flat straight place with no wind to test that.