I wonder if we're getting into the confusion of folks who think that "38C" is actually a tire size? And they're comparing a 700CX38mm tire to, maybe, a 650BX38mm tire? Thus thinking that writing 38C and 38B actually means something and is a basis for comparison. I haven't really figured out if this is the issue or not.
begin rant: I've given up correcting them, but I'll admit there's little that irritates me more than referring to a tire size in units of "C", not MM. "Duh, Ahm runnin' 25C's and am thinkin of runnin' 28C's"
Like already mentioned, the "C" refers to the wheel type, 700C. The 38 refers to the width of the tire, 38mm. Unfortunately, tires are printed as 700X38C which causes the ignorant to think that 38C is a tire size, and it's repeated until it virtually becomes truth, when it's meaningless without the 700C wheel descriptor (because, after all, there are 650C wheels fairly commonly used). What the heck do they think the "C" mean in a tire size? Surely, if asked they must think the C means something in regards to the tire size, but for the life of me, I can't imagine what they think makes sense for C to stand for. Centimeters? A 23, 28 or 38 CENTIMETER tire size? I'll leave it to you all to convert to inches. But even at the smallest of those numbers, we're in fat tire range, so the "25C" folks just can't be thinking of anything. I've even seen people talking about a 650B tire refer them as, for example, 38C.
End of rant
So maybe the OP is trying to compare a 38mm tire in 650B compared to a 35mm tire in 700C?
Last edited by Camilo; 08-13-22 at 11:50 PM.