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???