Changing 700x35C tires with 25mm? I'm riding on a hybrid bike with 700x35mm tires which is ideal in the city. Every now and then I could go on a ride outside of the city, covering distances in the vicinity of 80km and doing some medium climbs. I'm quite happy with how it all goes but I'm wondering what difference it would make if I change the tires to 25mm width? How much speed will I gain? Would it make the climbing a bit easier? And overall, is it worth it?

The tires are clip on so replacing them back and forth won't be such a hassle. I'm considering to have pairs of both 35mm and 25mm and replace them depending on the occasion but I would like to hear some feedback first on how wise that would be.

EDIT: Just to clarify, I'm planing to replace the whole wheel - both the rims and the tires!

Thank you!
Well, it'll depemd on how the narrower tyre sits on the rim, specifically whether a 25mm tyre will be wide enough, but as long as this is ok, you should be fine. Be aware that wider tyres will have a larger surface in contact with the road, and therefore more grip, but if you're happy to trade that off, go for it.