Originally Posted by
trailangel
I actually do this..... I know what gear inch I am riding at any time. It's always been a way for me to calculate mph while riding. Never had a speedometer and don't use strava or a gramin or any electronic device. I usually ride in a certain gear inch per various roads, and it's easy to spin 90 rpm on the crank because that is one second of time per 1.5 revolution per crank.
You are way out on the tail of the distribution curve then. Most people aren’t going to keep up to 30 numbers in their head, know which gear they are in at any given moment, count the number of revolutions, count the seconds, and then plug all that data into this formula
speed = π * (diameter + (2 * tire_size)) * (chainring/cog) * cadence
And do all that on a constant basis since the speed is changing constantly based on a whole bunch of different factors while also dealing with the whatever the world throws at you while trying to balance a relatively unstable vehicle on a fairly narrow contact patch.
If you have more than one bike with more than one wheel size, you multiply the gear ratios that number of bikes. I have 8 bikes and the drivetrains are only duplicated in a couple of them. That’s at least 6 different gear ratio charts I have to keep in my head. That’s just not going to happen for me nor for the vast majority of bicyclist.