1. Rear mechs are gear number blind, right? So I can use my (spare) *10 speed* Ultegra in place of a seized *9 speed* Tiagra?

2. 9 and 10 speed jockey wheels are both 11 tooth, aren't they?

Cheers

part 1 – yes
part 2 – not a clue

Cheers, r_r.

Well I have a used 7 speed mech on 9 speed as long as the mech will cover the width it should be ok.Anyway just noticed you want to use 10 on 9 so that shouldn't be a problem and if 10 speed jockey wheels were a bit thinner they would fit in a 9 speed chain as well

Edric – cheers.

I thought I was right, but needed the comfort blanket of STW to reassure me.

Sadly, I'll have to tolerate the seized and badly indexed mech on tonight's chaingang, but will sort it over the weekend.

You will be absolutely fine. You could use a 9 speed mech on 10 speed too, it will work just fine, the jockey wheel thickness isn't an issue.