Originally Posted by
MyTi
If I am simply relying on speed, there are too many variables in play, such as headwind.
Nobody is telling you to train by holding constant speed. All they said was that a speedometer is the best way to know to know how fast you're going.
As far as training, the point is that the usefulness of data has diminishing returns. It's possible to build a training plan around cheaper metrics like heart rate, or even seriously wishy-washy crap like "perceived effort", and still see results. The main reason that ownership of a power meter is well-correlated with rider performance isn't that power meters make you massively stronger, it's that the strong riders who follow actual training plans are the people who buy power meters.