My current rims are 19mm inner width (HiFi MixTape). 25c RBCCs measure out at 27.8mm on those, so I would be surprised if the 28c RBCCs come in under 29mm. Maybe just get the 25c version if you get the IRC.

Also, my local shop says the GP5000 TLs are already backordered for a while.

Thanks. There are a few sites in Germany you can order the 5000 TL's. As long as I get them before mid-March I'm good.

I was really hoping the RBCC's were more true to size, but I guess not. I'm trying to squeeze the fattest tire I can fit for some rougher courses this year.

I could look at the Schwalbe Pro One's, but I've never had great luck with Schwalbe tires...

Bicycle Rolling Resistance's test said the GP5000 TL tire they tested was a 25c and measured 26.8 mm on a 17c rim. I'm assuming they mean a 17 mm internal width. For comparison though, the GP4000 SII 25c tire measured 27 mm on what I presume is the exact same 17c rim in their prior tests. So if you can't fit a 28c GP4000 SII on your bike, don't expect the GP5000 TL 28c to have much better luck.

The GP5000 (non tubeless) 25c version did measure in at 26 mm, so if you aren't going to run tubeless, then maybe you have a chance with the 28c version of that tire but it all depends on how it mounts on your rim which I assume is wider than the rim BRR used to test.

I'm currently seeing dates of late March/Mid April so you better find them in stock somewhere.

The only tubeless road tires I've found to be true to size are Bontrager (R2 or R3 TLR only come in 24 or 26) and the new Hutchinson Fusion5 series (which are otherwise unremarkable tires). I've had good experience with Schwalbe Pro One for pavement use, but for courses including dirt/gravel I think they and the RBCCs are too light and thin. I've used IRC Roadlite 25's with good luck on such courses.

Tubeless 700x25's on HED + rims are 26.6mm @ 75psi. They were fairly easy to mount, used a tire lever to finish them off. No air compressor necessary, also no sealant in there yet.