beng1

I never said any one "must" use any crank length. You are under six-feet tall, not that tall. I was 6'3" a lot of my adult life, might have shrunk a bit as am in my 60s now.

A larger, heavier leg will take more energy than a shorter lighter leg to move at the same rate and same distance, that is physics and engineering 101. So if the large and small rider are in condition to generate the same watts of power, the only way physics will let the larger rider go as fast and as long as the smaller rider, is if they can have a longer pedal-crank arm which they can turn at a lower rate, a lower rpm, but still get the same power to the rear wheel. Power equals rpm x torque, so you can have less rpm if you are a large rider and get the same power for the same amount of time only if you are given a larger lever or crank arm, which you can pedal more slowly.

In the last year I have ridden road bikes I own with pedal cranks of 165mm, 170mm, 172.5mm, 175mm and 180mm. I have tested all of these bikes over the same 12.1 mile, mostly level circuit a number of times, some of them dozens of times, in the last year.. The bike with 170mm cranks, which I rode most often, had 165mm cranks the first half of the riding season, and changing them for the 170mm units gave me the fastest time around the circuit to that date. Toward the end of last riding season, when I got a bike with 180mm cranks rideable again, I chopped two entire minutes off the time it took me to go around the circuit with the 170mm crank-equipped bike, dropping the time from 37.5 minutes to 35.5 minutes, and raising the average mph from 19.5 to 20.4. I could run a faster gear ratio around the entire circuit with the long cranks, something I could not do with the same bike with shorter cranks, nor any of the other bikes with their shorter cranks.

26 years ago when I was younger and ran a lot of TT events, I was always fastest with 180mm cranks too.