Originally Posted by
SoSmellyAir
According to this road.cc review the Mavic SSC Cosmic Carbon has an internal width of 19 mm.
Review: Mavic Cosmic Carbon 40 wheels | road.cc
1. While 28 mm tires will fit, the setup would not be aero.
2. My GP5K nominally 25 mm tires actually measure 27 mm wide when mounted on my CF wheels having an internal width of 19 mm and sit flush with the 27 mm external width of those wheels. Better, but still not optimal aero.
3. Optimal aero supposedly has the wheel slightly wider than the tire, i.e., 105% rule. As the review says, good results were obtained with 23 mm tires.
My SL8 came with the Roval Rapide CLII wheels and the front one has a 35mm external width (wide!) while the rear one has a 30mm external width. The wheels are said to be optimized for 26mm tires and it's said to be backed up by wind tunnel, software simulation and real life testing. This said, my rims are larger than my tires, and this, even if I swapped the 26mm tires for 28mm tires this year.