They ride a track with a Powertap, possibly with an SRM as well (in most recent tests), at a moderate speed (150 watts, resulting speed 23-28.5 kph) then use AnalyticCycling to extract a Crr, which they temperature-adjust using results from a control tire. This isn't the most scientific method possible, as they don't say how they adjust for Crr..
Sorry I missed this discussion last month, Dan. Thanks for posting. I was thinking about buying the magazine, but figured I'd see if there was any commentary.
And... it doesn't look promising... like the coast down tests they did. We all wish testing Crr was this easy, but unless they do a lot of runs and a lot of swapping and determine that the scatter is acceptably low, then the numbers don't mean much. A Challenge Roubaix 27 (a fat smooth riding open tubular with little puncture resistance, that gets rave reviews for ride quality) nearly as bad as a Gatorskin?
Any variation in body position and wind can have a big effect. If they assume that wind drag is constant, then they must prove that this is the case. Did they attempted to prove it?