Bike gear calculator: metres per pedal stroke, gear inches and speed

Gear chart: 50/34 with an 11-34 cassette

For a 700 × 28 mm wheel. Each cell gives metres per pedal stroke and, in brackets, speed at 90 rpm.

The formulas

• Ratio: chainring teeth ÷ cog teeth. 50 ÷ 17 = 2.94.
• Wheel diameter: the rim's bead seat diameter + 2 × tire width. On a 700C rim the bead seat is 622 mm, so with 28 mm it comes to 622 + 56 = 678 mm.
• Development: ratio × π × diameter. 2.94 × 3.1416 × 0.678 m = 6.26 m.
• Gear inches: ratio × diameter in inches. 2.94 × 26.7 = 78.5.
• Speed: development × cadence × 60 ÷ 1,000. 6.26 × 90 × 60 ÷ 1,000 = 33.8 km/h.

Development and gear inches are two of the three classic systems for numbering gears, according to Sheldon Brown's glossary. The third, gain ratio, also takes crank length into account.

How to compare two drivetrains

Look at both ends and at the steps between gears. An example with the sums, on the same 700 × 28 mm wheel:

The single 40 gives a little more room on a climb (2.03 m against 2.13 m) and loses at the top: at 90 rpm it reaches 41.8 km/h, against 52.3 km/h for the double. Which is worth it depends on where you ride. We give no recommended gear here: no single number fits all legs and all climbs.

A practical way to decide is to start from what you already use: the gear you are in on your hardest climb, and at what cadence. The calculator tells you what development that is, and you can compare it with the cassette or chainring you are looking at. If you change the cassette, also check the chain length.

In Pedalytics

Pedalytics is not out yet: it is listed as «Coming soon». Ride analysis shows cadence and speed along the route when your file has them, and each ride's setup records the cassette you were running. This calculator works without any of that, in your browser.