Originally Posted by
neil0502
That's because your worn chain has similarly worn your cogs on your cassette.
The new chain will NOT match the wear pattern OF your worn cassette
+1
Unless your cassette wear is spread more or less uniformly over a reasonable number of your cogs. If you ride most of the time in one particular gear you will be SOL on that one.
You need to catch the chain wear well before the 1% mark. Once it gets to 1% you might as well let both the chain and cassette go bad together. Then buy both. This will also accelerate wear on your chainrings, but they wear out much slower unless you let things get really bad.