Originally Posted by smashndash

I would go with the 28. I have a 25mm corsa control on my 23i rim and that tire has a carcass width of 70mm. The GP5k S has a 64mm carcass.

Too small of a tire = more sidewall cuts, more exposed rims to gravel, and potentially compromised handling. These tires are just not designed for such wide rims.

Also, there is almost no change in tire height with wider rims. Which means that the effective height of the "spring" (tire) is almost the same regardless of rim width. So going with a narrow tire on a wider rim is not going to give you the same cushion as a wide tire.

That being said, I highly doubt you're at any risk of a blowout purely due to the rim width. If all you care about is aero, go for the 25.