I am a fan of wider tires- don't ride less than 32's. As has been stated I find higher quality rides nicer. Though I think width and the higher weight do affect efficiency.
If you sign up for BicycleRollingResistance.com professional service you can change the rates to match your weight and speed. GP5000's go from 10 watts to 24 watts for me. I think this is nice to know though having a consistant method provides a better tire to tire comparison.
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