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Feel like it depends on your tolerance for mud a bit and the actual shape of the tyre on the rim. There are pics on the internet of people wedging 1.9 and I'm sure I saw some 2.1s in somehow.
I also intend to try this this winter. But I have a sense that maybe the actual answer if you want a CDF with 650b tyres is a Fugio.
https://www.bikeradar.com/features/genesis-croix-de-fer-853-br-builds/ this is a modern CDF frame clearing 47s, just about.
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I have been running Hunt 650b on my 2014 for the past few months.. Started with WTB 47mm Byways, but they weren't giving me the grip I was after offroad.. so changed to 1.9inch Gravelking SK's, although these measured up at 52mm which was too tight for the CDF at the back, so just swapped out to 1.75in Gravelking SK's which measure 43mm, so a good 5mm clearance at the back and no issues.. pic below after 22 mile test ride at the weekend.
Taken it to a 10 mile ride, mixed surface. No rub. While riding looked down on the clearence. It stays the same. The wheels are new but according to Techinics100 he did not experienced any on the 2014 model with the same level of clearance and a long time user.
I will have an update as a long time use to see how it behaves.