I'm upgrading an older road bike that came with a Shimano 600 non-indexed derailer. I've installed a 105 8 speed STI shifter and an 8 speed cassette. The problem is that some shifts are correct, while others skip a cog and go to the next. I've spoken to several bike mechanics and they said my derailer should work fine since the indexing is done at the shifter. Here's what I found; The cable travels a toltal of about 7/8" out from the shifter to go from 1st to 8th gear. The derailer, however, requires only about 1/2" to 5/8" cable travel to move from the smallest to the largest cog. This results in the derailer going the full range of gears before the shifter has had a chance to advance it's full travel and so, it seems that maybe the ratio of derailer cable travel to derailer movment is not what it should be for this shifter setup. Bottom line; Do I need a new derailer? Are there varying ratios for different derailers?
Thanks,
Rich

They are full of it. Deralier does have to be matched to the shifters.Your derailer is pre index,and has no clue what the shifters are telling it to do.You need a new derailer.Any Shimano index capable(sis) 7 or 8 speed other than pre 9 speed dura ace will work.Any 9 speed shimano will also work.I cannot beleive such people call themselves mechanics.

If it's jumping from one gear to the next, I'd be wondering about the condition of the cogs and/or chain. Are both of these components used? If so, they are probably worn out, and need replacement.
Otherwise, I'd keep adjusting the derailleur. Perhaps the derailleur is not compatible with the shifters-this can happen when you mix-and-match old and new components. Chances are, a 6-cog derailleur is not compatible with newer stuff.

A 6 speed index capable(sis) Shimano derailer will work fine even with an otherwise 9 speed shimano index system. If it is not a shimaino sis derailer it don't work.simple as that.

Well, that makes sense to me. The cassette and chain are new, the only thing left is the derailer. The indexed type probably has a longer cable travel (and a slower action) than the non-indexed type.
Thanks,
Rich