norbs wrote:I am at a loss to know how accurate Strava is.
Strava's "accuracy" will depend on all those factors you have mentioned, and then some. Actually, at 45 km/h, around 70-80% of the power is used in overcoming air resistance, so I would expect the rider's aero profile will become a dominant factor as the speed increases.
However, it's not about how accurate Strava is in absolute terms, but how consistent.
As I mentioned before, I was surprised at how power meters vary in their readings. I was told this was normal - the important thing is not the absolute numbers but relative numbers over time.
The main issue about Strava is not whether it's over or under estimating (and the power meter you are comparing it with may not be that accurate in absolute terms anyway). The issue is consistency - there is no way Strava power numbers will be as consistent or as reliable as a power meter in terms of relative accuracy. I wouldn't trust them as a training aid, but then I am a casual rider - I ride for fun, I don't "train."