Crank length

[rich p]

The thing is that I'm buying a new groupset and the cheap deal offers 170/172.5 or 175 (out of stock).
I've always used and been happy with 175's so my inclination is to go for 172.5 as being the nearest. It's supposed to be a relatively cheap build so I don't want to alter the spec on offer by buying extra components.

 

[GrasB]

Go for 172.5 then. You may notice your cadence increasing by around 1-2 rpm if all goes well.

 

[gwhite]

For a 31" inch leg the crank length would be 165mm.

 

[simon.r]

You have a specific issue here, having had a hip replaced. Any implications of that should trump others. Longer crank and slower cadence would mean less hip movements. Shorter crank would mean less movement each rotation.
You need advice off an orthopaedic surgeon or sports physio.

Not the same, but on a similar theme I can't bend my left knee beyond 90 degrees and find 165mm cranks make life much easier - "less movement each rotation". There's a very noticeable difference between 165mm and 175mm.