I have run an 11-28 or a 12-28 cassette on the 2 wheel-sets I have at home. For those two switch-overs, I know I don't need to change the chain length, and the rear derailleur adjusts (right?).

I need to get a new cassette, so I'm wondering if I move to a 11-25, will I need to alter the chain length if I switch between that and the 11-28 on the other wheelset?

I have a race coming up that has almost no climb at all. Was wondering if the 11-25 will be a better choice for such rides, with availability of greater gear selection in the middle...

It should work too long of a chain is not a real problem unless it is really way long.

If the chain is sized so that it can do big/big and small/small with an 11/28 cassette, then it will also work with an 11/25 cassette.

The big/big will have slightly more slack which won't be bad.
On small/small, is there adequate tension in the chain? Does the chain rub on itself?

If not, you'll be just fine with the old chain length.

Make sure your chain is good, or go ahead and install a new chain with a new cassette.

You may choose to adjust the B-Screw so the derailleur lands closer to the cassette for more precise shifting. Or, if it works, don't worry about it.

The front stays the same at 52/36. The smaller on the rear also stays the same. I was just worrying if the change from 28 to 25 on the rear larger cog will make too much of a difference to make the chain slack.
The chain is not new/old (well, it's KMC X11 SL, with about 8000 clicks on it, so I believe it still has life in it). It's well kept though.

Javascript Bicycle Chain Length Calculator

If you use the chain length calculator above, you'll notice that changing from a 28 to a 25 in the rear makes almost no difference to the calculated chain length. You'll be fine.

If we knew WHICH RDER you have, we could maybe prove you don't need to shorten.

Cage size has no effect on chain length, so all the proof you need is given in the chain length calculator I linked to above.

For a 17.25inch chainstay:

52T/28T = 56 links (55.7in rounded up)
52T/25T = 56 links (55.01in rounded up)

Whatever the maximum differnence is likely to be no more than a single link, so no big deal.

You would actually ROUND UP over .01"?
You just blew your credibility with your own post.
What if the chain stay was 17.24"?

Also cage length affects TAKE UP CAPACITY, which is the point of the discussion.

"blew my credibility" really?

Yes, it's standard practice to round up to the nearest chain link, if I was doing in this my head I probably wouldn't in that case, but it's a computer algorithm so it does.

I did say that the difference is going to be at most 1 link, which isn't worth worrying about. I'm pretty sure I have my bases covered here.

That must be inches (inner link + outer link). Otherwise it would be an awfully short chain.

I notice you didn't address the 17.24" CS length.

You should also learn the difference between a link and link set.

8000 miles? 8000 km?

Is this a long term use of the new cassette, or just for a few days?

I'd go ahead and put on a new chain with your new cassette. Much of your cassette wear will be due to a stretched chain, whether it is 1/32" or 1/16" per foot, or more.

If you're planning on using both 25T and 28T cassettes, then size for the larger cassette.

Link in this case means the inner and the outer link combined. I know that when Shimano sell chains they count the inner and the outer links seperately. Nomenclature issues I guess. I tend to think of the inner and the other as a single unit, because when you're shorting a chain, that's the units you work in.

Because I'm not rising to your petty nitpicking. I said "it's going to be one link" at best.

Obviously you cant remove 1/2 a "link set", so it should have been obvious what I meant, but whatever.

If your chain works with your 28 big cog, it will work with your 25 one as well. It works when you are in your 25T cog now, right? No need for elaborate calculations.

A klick is a kilometer. A click(sic) is how some people spell klick.

Finally the correct answer.
Leave your chain alone, unless it needs to be replaced.

Wow, Got more replies in this thread than I anticipated.

Yes, it's 8000 km, not miles.

The RD is Dura Ace 9000, and they only come in one size.

I believe I got the answer, if it works with 28 big cog, it will work for 25 too. Thanks guys, for the input, suggestions and links.

Cheers.