But what are the lengths of the 2 cranks? I guess at 175 and 172.5 ... did I win?
I use 170s on my fixed road bikes, and only used a longer cranks fixed -- 175 -- on the brief (6 mos?) trial of riding fixed off road. I did install 175s on the Matthews ("road bike for dirt), but that has multiple gears so that I can upshift when I run out of rpm.
Actually, come to think of it, I did briefly use 172.5 Dura Ace 7410s on the erstwhile Ram, but didn't notice a difference. I do notice a difference between my 175 cranks and the 170s; 175 on the fixed gear mtb felt horrible when trying to spin -- it had a 60" gear, and downhills were torture; which is why I want back to 170s on that bike. And I do notice that I run out of steam at lower r's pm on the 175 mm Matthews than on the Rivs in the same gears.
But I wonder how much of this is due to Q and how much due to the 5mm of additional crank length; both the mtb and the Matthews have ~ 160 mm Qs while the 2 Rivs are at 130.
In any event, I'd rather grunt a bit more up hill in order to be able to spin easier downhill, thus 170s for my Rivs. Jury still out on the Matthews; may eventually swap the 175s for 170s.