For what it's worth, the newest pair of Schwalbe Marathon Pluses that I put on my wife's touring bike measured about one size smaller than stated, measured on two different rims. In other words the 38's that we purchased calipered at maybe 35 mm. But either way, you're running an unforgiving set up and hope you don't get a small piece of gravel to adhere to a tire via hot tar, as my wife once ruined her time trail bike that way when it wore a groove in the chain stay.