General isn't the right place for this either, IMO. Seems like a job for the Wheels and Tires forum--scroll down further and you'll see it.
I'll answer, though. For those who don't want to chase down the original thread, OP got a 23C and 25C in the same tire by accident, and wants to know if that's really a problem.
Let's assume that you can't just exchange one for some reason.
The difference will be pretty slight, but you might be able to detect it.
If you put the smaller tire on the front, you would ever-so-slightly quicken steering. Putting it on the rear will do the opposite. Somebody (Continental?) sells a set of tires like this just for the purpose of quickening steering--there's a 21 for the front and a 23 for the rear, I think. I have no idea if they actually do the job or not.
Personally, I doubt you'd feel a difference, but I'd put the smaller tire on the front--just knowing your tires aren't the same can have a strong mental effect, so you might as well put them on in the "faster" mode.
I've been riding with a 25 int he front and a 23 on the back for a few hundred miles now and have not noticed any differences. I considered reversing them but the 23 has a yellow band on it and looks better in the back.
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