Crank arm length?
 

I used to ride a 170mm crank arm then purchased a new bicycle and it came with 172.5 I got used to ridding the 172.5s. Then I purchased a used bicycle recently and it came with 170mm crank arms. I found with this bike I can spin better with the 170mm and maintain a higher average speed on the bike. I can also maintain a higher speed by like 5mph higher on the 170mm vs. the 172.5s??? My local bike shop mechanic said I should ride the 172.5s because it is a nickels worth of difference and you gain torque. I am 5'10 weight is 175 and inseam is like 29 inches. I only miss the 172.5 when standing and pedaling. What size is considered right? My bicycle is a 54 inch type. Is this like a saddle, whatever fells right works??? I purchased 170mm for my other bicycle and only notice the differance in the way I feel alittle further from the handlebars and seat is alittle high now. I can't tell a different in ride feel. I can keep a smoother spin and faster average speed on that bicycle also now, what is the deal???

It's a highly debated topic, and people's preferences vary (some like to spin short cranks, some like to mash long ones, etc.). I doubt a 5 mph difference is anything other than placebo, but 170 sounds about right to me. I'm 5'10 with a 31" inseam and I generally ride 170 or 172.5 cranks. In my racing days, I rode 165 or 167.5 on the velodrome and 175 on mountain bikes, so you can see where it depends on what you do and how you do it.