I plan to use my cyclocross bike for road cycling. In particular, I'll start in a triathlon, where I'll need to do 45km on the bike. I would like to maintain the speed of 30km/h throughout the race. The bike is equipped now with 700x32c tires (specialized borough cx). Many people say that I should change them to some 25mm. However, I've read on this forum that narrower tires have greater rolling resistance.

What tire would you recommend in my case? Note that I don't plan to go much faster, I'd rather like to keep 30km/h for the whole 45km and be as "fresh" after the ride as possible.

I'm assuming pavement? Road slicks in the 23-28mm range would be appropriate depending on your weight. But what rim width do you have? That can be a limiter, too.

The narrower=more RR has more to do with over-inflation than width. A tire that is too hard will bounce on uneven pavement instead of absorbing the irregularities.

yes, pavement. The rim is 622x15, and I'm 75kg

I'd go with some 25s, plenty of good light tires in that size.

I wish there were more nice 28s to choose from...

The 15 must mean internal width, and if so you should be able to run any traditional road tire. At your weight a 23mm at 100psi front and 110 rear is pretty standard, or go with a 25mm at a little less pressure if you like a smoother ride.

For tires, I don't know what your budget it, but Michelin Pro 4 Endurance are a good durable tire that still has a decent ride. I'd start there.

23c or 25c, PRO4 or Conti 4000s. The smoother the surface, the narrower the tire. Aero will be a bigger deal than tires. Train in as low a position as you can set up.