Originally Posted by
Retro Grouch
I don't think that you have that quite right. 7 and 8-speed cassette cog spacing are the ones within a fraction of a mm with one another. 8 and 9-speed varies by quite a bit and won't mix/match. Assuming OP has a 3 x 9 drivetrain now, if he wants to use an 8-speed shifter to save money, he'll need to buy an 8-speed cassette so I doubt he's going to save anything. I'd get the 9-speed shifter.
Shimano 7-speed is 5 mm spacing.
Shimano 8-speed is 4.80 mm.
Shimano 9-speed is 4.35 mm
So yeah, roughly twice the error mixing 8/9 as mixing 7/8.
Never said I thought it was a good idea.
But I've ridden with people claiming to be happy with it.
I might try it, If those were the parts I had home.
But I wouldn't buy mismatched parts like that.