Well. Mileage is mileage, but the power required to generate said mileage depends on the characteristics of the trainer and the road conditions. I can go 30 MPH down Wibble Hill at zero power but I can't generate sufficient power to do that on my Kurt Kinetic.
What would be interesting is whether one's FTP or numerous varients, such as the Carmichael field test, vary significantly from indoor to outdoor. You'd guess they'd be pretty close, but it would be cool to hear from anyone who has actually done it.
I only have an indoor power meter that derives the power value from the speed of the rotating cylinder on the Kurt Kinetic. It's accurate and reproducable, but when I ride outdoors I just kind of guess.
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Momento mori, amor fati.