I have a crank size concern here. The touring bike I am getting comes with a 175mm crank on the size frame I need. My bike now has a 170mm and is too small.

How much of a difference is the 175mm going to be?

I measured my inseam with the board/wall method and got about 31.5-32. According to the chart in the below link I should be at a 170mm crank.
https://www.cycleops.com/en/training/...it-part-4.html

I believe my BMX bike is a 180 or 185, if I had it here I'd check and ride it to see what it was like again...

What comments can you offer? I am not after speed. I'd say lower end gearing is going to be more important climbing hills, but the one bike I did ride (set up very similar - same model) gave me all the gearing options I felt I needed. I just want to match the way my legs work the best. It just seems the 170 on my current bike is too "tight" and I'm not getting the range out of my legs I should be. The pedestal is set right. Would 10mm (petal to petal) change over what I have now make that much difference?

10 mm is less than half an inch. I'm sure there will be posters who tell you it makes a huge difference to them but certainly not me. I once agonized about ordering a 175 mm crank when I was used to 172.5 mm and when it came I couldn't tell ANY difference.

Sometimes 1 measurement is worth 1,000 beliefs. I'd suggest measuring your BMX crank. I doubt it's 180 mm.

I used to think it would be hard or impossible to perceive a 5mm difference in crank length, but I noticed it when I wasn't looking for it. I have short legs, and I can't stand anything over 170mm. My pants inseam is 30" and my PBH is 33". I'm 5'9" tall, so my legs are short for my height.

How tall are you, and what sizes are your new and old bikes?

I have a feeling that the trend towards longer cranks is misguided. We all may be riding cranks that are too long for us. I've been collecting 165mm cranks, and they feel good to me. I'm sure a lot depends on style and taste.

Well,when it comes to bike fitting, even half of an inch can make a huge difference in a long run. You probably won't see much difference during short trips for pleasure, but if you are planning on racing, commuting, or riding long distances, I suggest investing in a good bike fit.
If you near SE Michigan, it's well worth to stop by at Welcome to Kinetic Systems Bicycle.com! They are located in Clarkston, MI. Jeff is the guru of bike fitting and all cycling related stuff. If you not close by /you didn't mention your location/, many of local bike shops can do that for you.

I ride bikes with 170 - 175, and the difference isn't noticeable to me, but the difference in the bikes themselves may mask the issue. Also, my ideal size (as I recall) is 172.5, so bumping up or down 2.5 mm is fine with me.

I can definitely sympathize with noglider that sometimes the amount of knee bend is the issue... when my knees are stiff, I know one of my bikes that I converted to rail trail use from a mountain bike is more difficult for me. Probably due to the 175mm cranks. A rider with short legs would just accentuate that.

I think it was the Sheldon Brown site (I read about it somewhere quite a while ago) that riding style also comes into play...

If you are a masher, and tend to ride at a low cadence, then longer cranks can give you additional leverage.

If you are a spinner, and tend to ride at a high cadence, then shorter cranks mean your feet move less per revolution, and you could theoretically spin slightly faster.

So, my opinion, especially since you feel your current cranks are too short is to try it out. You can always swap the cranks later if it is unbearable.

I can't tell the difference.

180mm. I had a chance to measure it this evening. Ill ride them side-by-side next week and see, though they are totally different bikes.

I dont know what the top tube is of the bike i have been riding. It is a cheap mountain bike. Ill have to measure jt when i get a chance.

For medium riders, standard 170 and 175mm are both OK.
Only short and tall riders need worry about crank size.

Huh. I ride 150's with the same inseam. Spin, spin, spin.

I got my road/touring bike today and put 10 miles on it. The 175mm crank with the gearing and how the bike rides fits really well. The combination gives me more available torque, but seems to be a bit of a loss in the cadence department - 61 before to 58 on my ride now, though I picked up several mph. So with all the factors together I am not sure I can really tell a difference strictly due to the crank size change, but it appears to be a non-issue. All together it is a pretty big improvement over what I was riding so I'm happy.

Since you already have the bike, I'm not sure if my information will be any help, but I'll give it anyway. I have a 31" inseam. For normal riding on a road bike 170 or 172.5 work fine for me and I don't really know if I can tell a difference between them. I have one road bike and a cyclocross bike (which I mostly ride on a flat rail-to-trail) which each have 175mm cranks. That is uncomfortably long for flat land and spinning - I can definitely feel the difference, but it works OK onbig hills. My full touring bike and mountain bike both have 175's and feel fine, but they are each used in lower-rpm/high torque situations and I like the extra leverage.

From your comments above, if you could definitely tell the 170's were too small I'd expect the 175 to be about right. The slightly lower pedal cadence also sounds probable.

Sounds about avg to me. I'm 5'9" 31 and 34. My PBR is 12 oz.