Crank length

I used to run a 172.5 on the fast bike (thats now on the winter one) the fast bike has 175 and I do feel like I can push a harder gear a bit 'more' when really going for it on the flat.

In theory a longer crank gives more leverage against the gear. But take it too far and the 'circle' your foot goes through will be too large and cause knee damage.

Personally, I would get your new bike and I would ride it for several hundred miles without worrying about any of the stock kit. I would simply set it up and adjust until it felt right and then enjoy it.

If you find you are having issues after everything has been adjusted, go for a proper bike fit.

Crank length is more a personal preference issue than anything unless you are switching by big numbers. You can replicate the effect by almost as much by changing pedal/shoe combinations.

Put simply... Don't worry about it.

When I had my bike fit I was recommended shorter cranks (170mm down from 172.5mm), though I'm only about 170cm tall. I can't remember the exact reason as to why it was better but it was something to do with having the saddle slightly higher makes better use of your muscles. I'm more comfortable with 170mm than was with 172.5mm so I'm happy. I definitely prefer shorter cranks on mountain bikes as I find them significantly less susceptible to pedal strike and it sorted my knee issues out as well.

I'm 1.71m tall so by Obree's reckoning, I need 162mm cranks - can anyone where I can get nonstandard lengths and what price I should expect to pay?
Thanks

If you have a substantial leg length difference then experimenting with crank lengths can be very beneficial, especially if the difference in length is in the femur. After a nasty leg break resulting from a motorbike accident I now use a 165 left crank and a 175 right crank, this has greatly improved my position on the bike.

all the road bikes are 172.5 and my mtb runs 175 - have
to be honest and say I can't tell the difference !

There's never been a definitive conclusion on what difference crank length makes. For example, Graeme Obree reckons 9.5% of your height is a good guide for a crank length. If you are 5'10'' - 11'' this makes 170cm about right which as a general rule is shorter than perceived wisdom. The important thing is that if you have more than one bike that they are the same.

http://www.stevehoggbikefitting.com/blog/2011/06/crank-length-which-one/

It's all to do with leg length, so manufactures, assume as a rider gets taller, their legs get longer, therefore increase the length of the crank arms (from the bottom bracket, to the pedal spindle). And vice versa, shorter riders get shorter cranks.
The only way to see if it is a major problem for yourself is to go and test ride the said machine.

Hope this helps,

Rob
@we8ster