Wheel circumference defaults to 2105 mm, typical for a 700x25c road tyre. A 700x28c is closer to 2136 mm and a 650b gravel tyre considerably less.
See which gears you actually use WattLog.PRO records cadence alongside power and speed for every ride, which shows you where your gearing really sits - and whether that climbing gear is doing any work.
Open WattLog.PROFour ways to describe one gear
Gear ratio is simply chainring teeth divided by sprocket teeth: a 50 tooth ring with an 11 tooth sprocket gives 4.55, meaning the wheel turns four and a half times per pedal revolution. Multiply that by wheel circumference and you get development - the distance travelled per pedal stroke, about 9.6 metres for that combination.
Gear inches is the older measure, expressing the gear as the diameter of a penny-farthing wheel that would give the same development. It survives because the numbers are familiar: 120 inches is a very tall top gear, 30 inches a very low climbing gear. All four figures describe the same thing; use whichever your riding group speaks.
Choosing gearing
The question that matters is what your lowest gear gives you on your steepest local climb. Work out the speed you can realistically climb at, check the cadence that produces, and if it falls below about 60 rpm you are grinding - which costs muscular endurance and, over years, knees. A larger sprocket or smaller chainring fixes it.
Top gear is usually less important than riders assume. A 50x11 at 90 rpm is already over 50 km/h, which is beyond what most people sustain outside a descent or a sprint, and on a descent you are coasting anyway. Gearing choices are nearly always better spent at the bottom of the block than the top.
Frequently asked questions
What gearing do I need for steep climbs?
Aim to keep cadence above 60 rpm on your steepest regular climb. For most riders on gradients over ten percent that means a ratio near 1:1 or lower - a 34 tooth chainring with a 34 tooth sprocket, or a sub-compact setup. Work backwards from the climb rather than from convention.
What is the difference between gear inches and development?
They measure the same thing in different units. Development is metres travelled per pedal revolution. Gear inches expresses the gear as the equivalent penny-farthing wheel diameter. Gear inches is traditional in Britain and the United States, development more common in continental Europe.
How do I find my wheel circumference?
Most accurately by rolling the bike one full wheel revolution with you on it and measuring the distance, which accounts for tyre compression. Otherwise use a reference figure: 2105 mm for 700x25c, 2136 mm for 700x28c, 2168 mm for 700x32c.