Gear development: metres per pedal revolution

What development is, and why metres beat gear inches

Gear development is the distance the bike travels during one complete turn of the cranks:

development (m) = chainring teeth ÷ cog teeth × wheel circumference (m)

A 50-tooth chainring on a 17-tooth cog gives a ratio of 2.94. On a wheel with a rolling circumference of 2,096 mm, that is 2.94 × 2.096 = 6.16 m per pedal revolution. Speed then follows directly: speed (km/h) = development (m) × cadence (rpm) × 0.06. At 90 rpm, every metre of development is worth 5.4 km/h.

Gear inches (ratio × wheel diameter in inches) date from the high-wheeler era and are still quoted by some manufacturers and by British touring writers. They work, but they describe a diameter, not something you feel, and you have to convert before you can relate them to speed or to your cadence. Metres per revolution are the distance you actually cover, they multiply straight into speed, and they let you compare a 650B gravel bike with a 700c road bike without a calculator. They are also the unit GearWatts uses everywhere it shows a gear.

One caveat on the wheel: the rolling circumference depends on the tyre and its pressure, not on the nominal size. A 700×25–28 road tyre measures roughly 2,100 to 2,140 mm when rolled out under load. The figures below use 2,096 mm; with a slightly larger real circumference, every development is about 1–2 % higher.

Common setups side by side

The table lists the hardest and easiest gear of three typical road drivetrains, with the speed they give at a steady 90 rpm.

Setup	Hardest gear	Speed at 90 rpm	Easiest gear	Speed at 90 rpm	Ratio range
50/34 with 11-34	50×11 = 9.53 m	51.5 km/h	34×34 = 2.10 m	11.3 km/h	455 %
52/36 with 11-30	52×11 = 9.91 m	53.5 km/h	36×30 = 2.52 m	13.6 km/h	394 %
48/35 with 10-33	48×10 = 10.06 m	54.3 km/h	35×33 = 2.22 m	12.0 km/h	453 %

Two things stand out. First, the three tops are within half a metre of each other, so on the flat nobody is really limited by a 48 instead of a 52. Second, the 10-tooth cog buys a smaller chainring almost for free: 48×10 is harder than 50×11 and the 35 gives an easier bottom gear than the 36.

Real life is rarely at 90 rpm. Spinning at 100 rpm in the 50×11 means 57 km/h; most riders run out of legs or road long before that. If you almost never use your hardest gears, you are carrying gearing you do not need.

Steps between cogs and overlapping gears

The jump from one cog to the next is the ratio of their tooth counts. On a typical 11-34 cassette (11-12-13-14-15-17-19-21-24-27-30-34) the steps are:

• 11→12: 9 %, 12→13: 8 %, 13→14: 8 %, 14→15: 7 %
• 15→17: 13 %, 17→19: 12 %, 19→21: 11 %
• 21→24: 14 %, 24→27: 12.5 %, 27→30: 11 %, 30→34: 13 %

A step of 7–8 % is barely noticeable; one of 13–14 % forces your cadence to drop from 90 to roughly 79 rpm if you keep the same power and speed. That is why close-ratio cassettes like 11-28 feel smooth on the flat, and wide ones feel jumpy exactly where you are tired, on climbs.

Overlap is the other half. With 50/34 the chainrings differ by a factor of 1.47, and a cog step is about 10 %, so when you change ring you need to move four cogs to land on the same development. If your small-ring gears are all faster than your big-ring easy ones, you have overlap: the same development exists twice, which is wasted range. If the gap is bigger than the cog steps, you get a hole in the ladder at the moment of the front shift. A 48/35 pair (ratio 1.37) needs only three cogs of compensation and is easier to ride because of it.

Choosing chainrings and a cassette for your terrain

Start from the easiest gear, because that is the one that decides whether you finish the climb. Take a 78 kg rider and bike on an 8 % slope at 11 km/h: gravity and rolling resistance ask for roughly 205 W. In 34×34 (2.10 m) that speed is 3.06 m/s ÷ 2.10 m = 87 rpm, comfortable. In 36×30 (2.52 m) it is 73 rpm, still fine on paper but heavy when the slope stays at 8 % for 40 minutes. At 12 % and 9 km/h the 34×34 gives 71 rpm; the 36×30 would be at 59 rpm.

A practical way to decide:

1. Pick the steepest sustained slope you ride regularly and the speed you reach on it at your usual power.
2. Convert speed to metres per minute (km/h × 16.67) and divide by the cadence you want to hold (say 75–85 rpm). That is the development of your easiest gear.
3. Do the same at the other end: your top speed in a group on the flat or a descent where you still pedal, at 95–100 rpm.
4. Choose the chainring pair and cassette whose endpoints cover that range with steps under about 12 % in the middle where you spend your time.

Hilly regions with long 8–10 % climbs usually point to 50/34 or 48/35 with a 33–34 cog. Flat or rolling terrain where the group sits at 40+ km/h favours 52/36 and a tighter cassette. Neither is "better"; each trades the range at one end for finer steps in the middle.

Checking what you really use

Theory is a start; your rides show the rest. If your head unit records gear changes from electronic shifting, GearWatts shows time, power, cadence and speed for each chainring × cog combination, with development in metres per pedal revolution and a view of the gears you choose at each gradient. See what electronic shifting records in your ride files for how that data is obtained and read.

Two patterns are worth looking for. If your easiest gear is used for only a few minutes per month, a harder bottom end would give you finer steps for the rest of the cassette. If you are often in the last cog with cadence under 60 rpm on climbs, your gearing is too hard for your terrain. Measured climbs, with their gradient and category, are explained in how climbs are detected and categorised; they are the right place to look because a climb's slope is steady enough to compare gears fairly.

Finally, remember that gearing is cheap to change compared with fitness or weight. A new cassette costs a fraction of a lighter wheelset, and one metre of development at the bottom end can be the difference between grinding at 55 rpm and spinning at 75.