I need some help guys. Anybody who is 5'8" or close what length do you have on your crank arm??..i just bought myself a shimano 105 for just $120 and the length is supposedly my length which is 172.5, but i want to be sure.

There are many length calculators that you can google.
Your selection may be right or a bit long, depending on your inseam measurement.
But everyone's different, and the only true test is experience with different lengths to compare them.

I am a bit statuesque at 5'11" and I perfer a 172.5, I am not a good spinner and the 172s always felt better. wheter it is the "correct" crank length for my height and leg length I have no Idea, I just tried a set and I seem to be more comfortable on them.

but I agree with Metzinger there are several methods to caculate this.

I'm 5'8, 30" inseam, and I have a 175mm on my mtb, and my Specialized Sirrus came with 170mm cranks. I sure as heck think there is a difference, but might just be me. The Allez I used to own had 175mm as well, and I feel the 175mm was easier to pedal?

172.5 should be fine.

6'1 with different setups, 175 and 172.5. Over 60,000 miles, 35 or so centuries and I stil can't tell a difference in crank length.

1 mm = .039...so 2.5 X .039 = .097 of an inch which is less that 1/10 of an inch. If that affects your pedal stroke, you are very sensitive. That's less of a difference than inserting and insole which many riders do.

It would make a differnce in lenght if it were a dude 6'7 and 180 cranks vs a 5'2 with 172.5. Then you are talking a difference, but other than that.....Pfft!

I've read one statement that said a 172.5 corners better than a 175. Sure, if you're dumb enough to take a sharp turn and lay the bike down on it's side with the pedal down!

I prefer a 170, I am a spinner and prefer the smaller diameter as it is easier for me to turn high rpm smoothly. That said, two of my four bikes have 175s on them. Near 5-11 and 86cm PBH. Go figure.

175, 172.5, 170. You're only talking about around 1/4" difference on the outside. I doubt very many people of average size (5,8" is roughly average) will notice a difference. I know I can't.

When I bought the compact crankset for my road bike I worried endlessy because I cound get 175 mm immediately and 172.5 was going to be an indefinate wait. I bought the 175 and couldn't tell any difference.

I think you will be OK. Might be a bit long for you, might be just right. Besides, since you already bought it you might as well give it a try.

I use 170s on my geared bikes and 165s on my fixed bikes.

If you're riding fixed gear you may not have a choice. That's why I use 165s.

That's true! But the sharp turns I had in mind were 35-40 mph swithcbacks on mtn descents, aint happening on a fixie!

I do know that some flip flop wheels may have a freecoasting side (?) so if a fixie were to do sharp turns on the ride I describe, they have the ability to keep the pedal up in the turn.

We've had some excellent fixie riders do 5,000 ft climbs with us. They are fine on the way up but never do they keep up on the way down. Even with the freecoasting (?) side, gear isn't big enough to keep our speeds. I imagine they could put a larger gear in the rear, but still...... Plus "most" fixie riders don't do these climbs.