Originally Posted by
Steve Whitlatch
I have run 8 speed 105 STI levers with a 7 speed Shimano cassette. All you need to do is use an alternate cable routing as shown in Sheldon Browns site. Shifts fine.
I thought (okay, I think) the spacing is the same. The only difference is that the 8 speed shifters have one more click and can move the cable 14.3% farther.
At any rate, it's very reliable stuff. This is what my most modern bike runs.