Will I have a low enough gear for this 9% climb? A big enough one so I don’t spin out on the descent? How many genuinely different gears do I actually have on my bike? You usually answer these by feel, or by swapping a cassette to find out. 2PEAK’s new Gear Calculator gives you the answer in black and white: for every chainring-and-sprocket combination, it shows the gear ratio, the rollout and your speed at a given cadence — and it flags the gears that duplicate each other and the combinations to avoid. Let’s look at what it does and how to use it.
Gear ratio, rollout, speed: three ways to read the same gear
Three terms to pin down first, because they come up everywhere.
The gear ratio is your chainring’s tooth count divided by your sprocket’s. A 50 × 12, for instance, gives a ratio of 4.17: for every pedal stroke, your wheel turns 4.17 times. The bigger the number, the harder and faster the gear; the smaller it is, the easier it is for climbing.
The rollout (or development) turns that ratio into real distance: it’s how many metres you cover with each pedal stroke, found by multiplying the ratio by your wheel circumference. With a classic road tyre (700 × 28C, about 2.14 m around), that same 50 × 12 carries you nearly 8.9 m per stroke.
Speed, finally, is simply rollout times your cadence. Those 8.9 m at 90 rpm come to roughly 48 km/h. Change the cadence and the speed follows — which is exactly why it helps to have that dial to play with.
One thing about the wheel: its circumference depends on your tyre, and the calculator works it out from the ETRTO size (the standard that describes a tyre by its width and rim diameter — say 28-622 for a 700 × 28C road tyre, or 42-622 for a 700 × 42C gravel one). A bigger tyre is a slightly larger wheel, so a slightly higher rollout and speed for the same ratio.
What you enter, what you get
The idea is straightforward: you describe your drivetrain, the tool works out the rest.
First you enter your chainrings — one, two or three — and their tooth counts (order doesn’t matter, it sorts them for you). Then your cassette: either you give the number of sprockets with the smallest and largest, and the “Calculate sprockets” button spaces the teeth in between; or you type the exact list of your sprockets yourself, freely editable. You pick your tyre from the ETRTO list, set a cadence, and that’s it.
In return, you get the table of every possible combination, each with its ratio, rollout and speed. In a single screen you see your bike’s full spread — the gap between each gear, your range from easiest to biggest — instead of guessing at it.
Spotting the duplicates
On a two- or three-chainring setup, a good share of the combinations overlap: a big chainring on a big sprocket can give almost the same ratio as a small chainring on a small sprocket. On paper you might have “22 speeds”, but the number of genuinely distinct rollouts is often far lower.
The Gear Calculator marks these overlaps. That’s useful for two reasons. First, you finally see how many really different gears you have under your fingers. Second, you know when it’s worth shifting the front chainring rather than working through the sprockets: no point hunting on the big ring for a ratio you already have, almost identical, on the small one.
Avoiding extreme cross-chaining
When you pair the big chainring with the largest sprocket (or the small chainring with the smallest), the chain runs at a sharp diagonal across the drivetrain. This is cross-chaining, or a poor chainline, and the tool flags these combinations as ones to avoid.
It’s not just about noise. A crooked chain rubs against the teeth and the derailleur, wears faster, shifts less cleanly — and costs you a little efficiency. Lab measurements show that chain-drive efficiency isn’t fixed: it varies above all with sprocket size and chain tension, from near-perfect in the good cases down to markedly heavier losses in unfavourable ones (Spicer et al., 2001, Journal of Mechanical Design). The practical takeaway is simple: for a given ratio, favour the “straight” combinations in the middle of the cassette, and skip the extreme crossed ones.
What it’s actually good for
The tool really shines at decision time. Before buying a cassette or new chainrings, you can compare two setups and check, with the numbers in hand, that you’ll have a gear easy enough for your climbs and big enough for your descents. You can compare a single-chainring setup (1x) with a double (2x) to see what you gain in simplicity and what you lose — or don’t — in gear spacing. And since speed is shown directly, you know in advance how fast each gear will take you at your usual cadence.
In practice, you’ll typically use it to:
- work out your climbing sprocket: the rollout you need to keep the legs turning on a given gradient;
- compare setups (1x/2x/3x, wider or narrower cassettes) before you invest;
- know your top speed at a realistic cadence;
- fit the calculation to your terrain by simply changing the tyre (road, gravel, MTB), since circumference is part of the equation.