Originally Posted by
macca33
FWIW - I've got a pair of Conti CX Speed 35mm on one wheelset (for gravel / dirt / CX, etc) and a pair of Conti GP4000s (23mm) on one set of wheels for road / tarmac, when commuting on my Focus Mares. Interestingly, the Conti CX Speed tyres do roll very nicely on tarmac - they are just heavier and wear quicker.
GP4000s in 23mm would suit you fine - for tarmac. If you do venture onto gravel / dirt, then a CX tyre would be more appropriate, I'd reckon.
cheers
The CX Speed/Speed Ride tires do roll really well on pavement, but I wouldn't want to use them for pavement only. They are great for mixed pavement/gravel ride.
For the OP's actual question, you should be able to run 23's or 25's on your CX bike without a problem. The only potential issue putting small tires on a bike designed for fat tires is that the pedals will be a bit closer to the grount, meaning you could hit the pedal on pavement while cornering. However, most CX bikes have a pretty high bottom bracket, so I don't think this should be an issue. I used my old CX bike for commuting all the time, on 25c tires, and it was great. I just had a spare set of wheels with the 42's for riding gravel.
If you are using this bike for pavement-only rides, definitely go with a 25c tire. IMO, that is the best size for pavement-only riding. 28's or 32's would be ideal if you have especially rough pavement or might ride a bit of gravel, but I think 25's are the best for tarmac-only riding.