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I would like to join some CX races that I kept missing time for in the last few years, that is probably the main and only reason for staying fairly narrow.
Leaving the knobby questions for now, do you think for tarmac and maybe occasional gravel (considering having a second wheelset for harder terrain) I should go for 650Bx42-48 or 700x28-32c? My 700c rims are 20 mm inner width so won't go any narrower than 28c.
Is mud that big of an issue for you? Personally I find actual mud doesn't feature very often... CX tyres might be better on mud but actual gravel or dirt, rocks, roots, etc. you're better off on fat tyres. With aggressive knobs you can do mud too - just look at MTB. I have 700c/650b wheelsets too but view it more like road bike tyres and verging-on-MTB tyres, in which case you want fat off-road and narrow on road
If the stuff you're doing is more CX than gravel then I guess it makes sense to have 700c knobblies though