Yes, inner rim width. To be honest, it probably is only a 35mm tire in actuality...never measured. I run exactly 65 psi in the rear, about 55-60 in the front. I've never had a problem and they are ridden from Alexandria, VA into the heart of DC every day....over 2 bridges and lots of crappy pavement.

I attached a couple of pictures of it. Your experience may be different, so I don't want to say it is 100% safe...but it works for me .