Trying to think of any reasons why not, neglecting the chain width difference potentially causing issues on the jockeys?

Can’t see how it would matter how far the shifter is telling the mech to budge, that’ll be the same regardless so no indexing issues?

It’s late, I’m tired, this may be simple but please clear it up for me STW

The only problem might be the RD shift ratio. The cable will be pulled a fixed amount by the lever, but different RDs will move by different amounts in response
This might help

Pre the latest shimano as long as both are shimano yes it will work.

No it won’t work assuming you are talking shimano MTB.

Up to 9 speed all mechs and shifters are compatible, but the pull ratio changed for 10 speed. It’s basically a question of leverage, a 9speed shifter might pull 1mm of cable for each gear so when the mech gets pulled 1mm it moves the 3mm needed to change one gear.

A 10 speed shifter might pull 2mm of cable to get the same result so if you connect it to an 8speed mech it’ll move 2gears not one.

(numbers for illustration only)

However a 10 speed Road mech will work with an 8speed mtb shifter.

Sram stuff will also work (as well as it ever does…)

No, 10 speed rear mech wont work properly with 8 speed shifters, not just the pull ratio though is it? Would only take a small difference in mechs pivot locations to change ratio irrespective of amount of pull at lever.

The mechs pull ratio is pretty much just the position of the pivots.

So yes. It is just the pull ratio.

And no. A 10 speed mech won’t work. 9 will be fine though.

Cheers all!

If you’ve got a 10 speed mech you are hoping to use I’ve got a 9 speed XT in spares box I’d be happy to swap for it.