I need a sanity check here, please help!

I want to fit a 10 speed road cassette to an MTB wheel. The wheel I’m intending to buy says “FORMULA 8/9 SPEED BLACK ALLOY CASSETTE REAR HUB”. That’ll fit won’t it? As 10 speed was no wider than 8 or 9 speed?

No-one? Surely someone can help!

Yep it’ll fit, you need the 10-speed spacer supplied with the cassette (as 10s is slightly narrower in fact than 8 or 9).

Should be fine, the width difference between 8/9/10 speed cassettes is very small. You’d probably struggle to fit an 11-speed on though.

As ever, Sheldon Brown is your friend:
http://sheldonbrown.com/cribsheet-spacing.html

Thanks all, you have proved I am not going nuts.

you need the 10-speed spacer supplied with the cassette (as 10s is slightly narrower in fact than 8 or 9).

Unless it’s a 10spd Tiagra / HG500 or SRAM cassette as these don’t use the 1mm spacer.