Originally Posted by PepeM

I have not. All I am saying is that between trusting a Stages power meter 'with all the things that can affect that' and trusting virtual power, also with all the things that can affect that, I would trust the Stages power meter. If in your particular case the virtual power was closer to a power tap, which makes it better in your eyes well, that's good.

In any case, the main point was that when looking at large discrepancies such as the one found by the OP, I would look first at the virtual power as the one more likely to be wrong. The Stages is a device designed and sold exclusively to measure power. Virtual power is an estimation offered for free by a software company, based on data supplied by a third party. Sure, the Stages has the whole left/right issue, but then the virtual power has many things to consider too: variability between trainers of the same model, ambient temperature, tire used, tire pressure, weight of the rider, etc., just as Nick posted up there:

Again, I am not suggesting virtual power is useless or always inaccurate or anything like that, simply that I would assume it to be wrong before assuming the power meter is wrong when seeing 40+ watt difference. I am sure there will be cases where the virtual power will be more accurate (such as the one you described, if we assume the PowerTap reading is the 'real' power) but I bet than more often than not, the Stages would be closer to the 'real' value than the virtual power.