Both are Specialized tires, DH's tarmac has Specialized Mondo and my Ruby has Specialized Roubaix. I did check the Specialized site and it says that 23 is the radius and the 25 is the casing. Still not exactly sure what that means, but the Mondo's are noticeably narrower. Don't know if I will go with another Specialized tire anyway, I am leaning towards the Vittoria Rubino Pros.