Many of us have a crank length that works best for us. I rode 170 my first several bikes including the bike I first raced. Mid-season I changed bikes to one with 175s. First ride was my morning training loop. I went deliberately easy so as not to pull a "surprise" on my knees. That easy ride matched my best time ever. 3 days later, two minutes came off that and continued to fall all summer.

I've stayed with 175s since. They have never not felt "right". Had I never ridden that bike I probably would nave muddled along for years never knowing things could be better.