my wife can tell the difference between a 172.5 and a 175 in less than a mile of riding. She's a 5'6 american longstemmmed beauty.
It seems that cranklength is more a function of thigh length than total leg length.
The above being said, you really need to try riding a 170, 172,5 and 175 to see which feels better.
The difference in length will not have much impact on gearing but more in your cadence and bike feel.
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