I tried fitting a 28mm Continental GP4000 on my skinny tire bike but they measured 30mm on CR18 rims and rubbed on my fenders. 28mm Gravel Kings are tight but so far so good. I might pick up a 32mm Continental 4 season to go with it, as they supposedly are closer to 30mm actual, and would be a more durable tire to put on the rear wheel, but then I would need to have add a fenderless skinny tire bike to the herd.