Gear inches, development, and gain ratio for cyclists
The first time somebody told me their bike had 'a 100-inch gear' I assumed they were kidding. A 100-inch wheel? Turns out gear inches is a real, useful unit — it's the diameter of the imaginary penny-farthing wheel your modern geared bike is equivalent to, and it survives because it's actually the most intuitive way to compare gears across bikes with different wheel sizes.
Bicycle gear inches = (chainring teeth / cog teeth) × wheel diameter in inches. A 50-tooth chainring driving a 25-tooth cog on a 700c wheel (27 inch effective diameter) = (50/25) × 27 = 54 inch gear. A pro's biggest climbing gear might be 25 inches; their biggest sprint gear 130+ inches. A single-speed commuter is often 65-72 inches.
Why this survives
Comparing 'a 50/12 with 700c wheels' to '48/13 on 650b wheels' is useless in your head. Both reduce to gear inches (112 vs 96 — the first is a bigger gear). It normalizes across wheel sizes, which matters more now that gravel bikes with 650b, 700c, and 27.5 inch wheels all coexist.
The metric alternative — development
Development is the distance the bike moves per pedal revolution, in meters. A 54 inch gear × π × 0.0254 m/inch ≈ 4.3 m per pedal stroke. Some parts of Europe use development instead of gear inches. Same information, different units.
Gain ratio
Sheldon Brown popularized gain ratio: (chainring / cog) × (wheel radius / crank length). It accounts for crank length, which matters for actual mechanical advantage. A person on 165 mm cranks and a person on 175 mm cranks running the same 'gear inches' actually experience different leverage. Not many people bother, but it's the most physically honest version.
Recap
Gear inches is a legacy unit that's still the shortest way to compare gears across wheel sizes. Under 30 = climbing, 60-80 = flat cruising, 100+ = descending or fast group riding.
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FAQ
What's a 'granny gear'?+
The smallest chainring on an old triple crankset — used for very steep climbs. Modern 1x drivetrains replaced triples with wide-range single chainrings.
Does wheel diameter really matter?+
Some. A 26-inch wheel with the same 50/25 gearing is (50/25)×26 = 52 gear inches vs 54 for 700c. Small but noticeable on long rides.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-23 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.