1 you measure, yourself, .. other you believe what you read online ....

I put Conti 28 on a Mavic e2 rim and they're measuring 25..

Insufficient Clearance? send the frame out to be indented carefully, for more clearance??

Only Sew up , Tubular tires, can control the tire width , ..Tufo tubular clinchers engage clincher rim's edges with 2 channel rings..

otherwise the rim choice effects the width, taking up a section of the circularity...





....

Depends on the rims. On my old Araya CTL-370 rims (14mm inside width) 700x23 and 700x25 Conti and Schwalbe tires measure right at their nominal widths. That's on an '89 Centurion Ironman, which was originally equipped with 700x18 or 700x19 tires on those rims. 700x23 feels like the sweet spot for handling and comfort. 700x25 felt a bit splashy on the front on fast curves -- not enough support for the wider tire.

Rims with 17mm inside width may be a bit too wide for an old school road bike like that. That's what I'd choose for 700x28, maybe up to 700x32 tires, but my road bike won't handle those.

Or, if not different and narrower rims, try some new/old stock Schwalbe One V-Guards in 700x23. They didn't run wide, and have more supple sidewalls than the Conti Ultra Sport 2. They're still available, around $30 each. Excellent road tires. A bit prone to nicks so avoid broken glass, etc. But even after many nicks and cuts mine kept going for months, long after I probably should have replaced them. Good puncture resistance for a lightweight tire with minimal puncture shield. Smooth and fast rolling too. Wish Schwalbe had kept making 'em.

I've switched to Conti Ultra Sport 2 because they'e a bargain for good performing tires. Heavier than the Schwalbes but much more resistant to nicks and cuts, and no more puncture prone despite lacking a puncture shield. Both the 700x23 and 700x25 measure right at nominal specs *on my rims*, per my calipers.

I second this. If you want to run narrow 23mm tires, you'd be better off running rims with something like 13mm inner width. I'm gonna guess that these are not the original wheels for this bike, right? Sounds like you have some decisions to make.

I've had to dimple the chainstays on a few frames for additional clearance. Sometimes you can get by with just a little bit of strategic flattening. This is an option, but I'd still recommend narrower rims.