I ordered a 10 speed XT cassette for my 29er from Halfords (less than £26 with all the discounts). However! There was no size choice and the pic showed an 11:36t cassette, so I assumed that’s what I would be getting!! However! I have received a 11:32t cassette. I need my bike for Sunday’s Scott MTB Marathon in Wales, hence the question. I have a 39/27 chainset up front, in the real world would I actually notice a difference?

Yes you would notice… unless your fit. You’ve lost over 10% on the lowest ratio. 27 is a big small ring for a 29er anyway (22 or 24 would be more common), and then thats paired with a 32 cassette – makes for a big smallest gear. That said, I ran 34/34 for the longest time without a problem, which is much bigger than your gearing…

Can you not just return it to Halfords (when you get the chance) and buy the correct version before tomorrow ?

I don’t know much about that event, but it’s possible there are some bike component retailers running stalls, and you might be able to buy a 11-36 there ? The event organisers should know which traders are turning up, and if it’s a bike shop, you could phone them today.

Considering it’s a marathon, 27×32 might hurt a bit on a 29er. However, when you’re that knackered after many hours riding, don’t forget that getting off and pushing up steep hills isn’t necessarily any slower. It depends how much climbing there is though….