Old man muttering...
"Gear Inches" = D (Wheel Diameter in Inches) * (Crank Teeth/Gear Teeth)
Example using easy numbers:
- 27" * (30/15) = 54 Gear Inches
- For every full crank rotation, my 27" diameter wheel makes two revolutions
- But I don't traverse 54", I traverse 2*(27*3.14) = 170" (rounded)
- Although "Gear Feet" would be more practical (14.1 in this example)
So "Gear Inches" is only useful as a relative reference point when you are talking to other cyclists. They know that road bikes range from about 30 to 127 (excepting granny gear folk like me). My range, using 700x25:
- 53/11 = 127.1 Gear Inches
- 30/34 (Yeah, I've got L-O-W gears!) = 23.3 Gear Inches
- So I should remember for those rare occasions where I am talking to a gearhead, my Gear Inches range from 23 to 127 - keep repeating until it sinks in, 23-127, 23-127...
In reality, MPH @ 100 RPM is much more useful:
- 53/11 = 37.8 mph @ 100 rpm
- 30/34 = 6.9 mph @ 100 rpm
Using this, since 20% is easy to add or subtract, my cadence is seldom under 80 and my absolute max so far is 120, I can calculate:
- My absolute max speed (45* @ 120 rpm on the steepest downhill I've found).
- My slowest climb at 5.5 mph (20% under 6.9)
There have been instances where I was going slower, but I expect to banish that this coming year thanks to my new hip, new knee, and improved attitude!
* FWIW, that's the highest speed I've ever observed. Unfortunately my CF Roubaix did not feel solid - it verged on a shimmy.