Originally Posted by
Steve B.
And yet I’ve read that you get a better aerodynamic profile with a wide rim and tire near the rims width. So somehow that works.
Most 25mm rims have an overall width greater than 30mm.
Recalling a conversation with a very experienced wheel builder, the advantage is a properly matched rim/tire vs a tire with a too-small rim. That would cause a “lightbulb” profile, which has a poor aero shape.
Even if there is no aero advantage, you are still at higher risk for pinch flats.