I don't know about science, but I've ridden 170, 172.5 and 175 on 52-55
frames. What I have found is that with 170s I can spin fast, but don't have
the leverage for hills. As I have gotten older, I don't spin so fast either.
With 172.5, I can still spin and have good leverage for hills. With 175, my
knees come up too much, causing me knee problems. So, I ride mostly 172.5
but I still have a couple of bikes with 170 as well. No 175 on my road
bikes. Now that I live in Florida and don't have many hills, I'm finding
that my 170s aren't a handicap like they have been other places I've lived.