What a gear ratio actually is
A bicycle gear ratio compares two sprockets: the chainring at the front, which the pedals turn, and the cog at the back, which turns the wheel. Divide the number of teeth on the chainring by the number of teeth on the cog and you get the ratio. It tells you how many times the rear wheel turns for each full turn of the pedals.
A high ratio, often called a tall or hard gear, means the wheel turns many times per pedal turn. A low ratio, a short or easy gear, means it turns fewer times. Bikes with several gears simply let you change the ratio by moving the chain between different chainrings and cogs, which is the basic idea behind how bike gears work.
A worked example
Take a front chainring with 50 teeth and a rear cog with 15 teeth. The ratio is 50 divided by 15, which is about 3.33. The rear wheel turns roughly 3.3 times for every pedal turn.
Now take a chainring of 34 teeth and a cog of 25 teeth. The ratio is 34 divided by 25, which is 1.36. The wheel turns a little more than once per pedal turn.
To see what that means on the road, we need the distance one wheel turn covers. A common road wheel and tyre travels roughly 2.1 metres per turn, though it varies with tyre size. Using that figure:
- At a ratio of 3.33, one pedal turn moves you about 3.33 x 2.1, or roughly 7 metres.
- At a ratio of 1.36, one pedal turn moves you about 1.36 x 2.1, or just under 3 metres (about 2.9).
If you pedal at 80 turns per minute, the first gear gives about 7 x 80 = 560 metres per minute, which is about 34 kilometres per hour. The second gives about 2.86 x 80 = 229 metres per minute, which is about 14 kilometres per hour. These are rounded illustrations that assume a typical wheel, not exact figures for your bike.
The trade-off: top speed against acceleration
The tall gear takes you further per pedal turn, so it can support a higher speed without spinning your legs very fast. The price is that each pedal turn needs more force. That is why a high ratio feels heavy when you start from a stop, climb or ride into a headwind.
A low gear is the opposite. It takes little force per turn, so it helps you accelerate and climb, but you run out of room to go faster because your legs would have to spin impractically quickly. Neither is better in general. The right gear depends on the terrain, the wind, your fitness and the speed you want. The reasons are tied to how a bike accelerates and how effort relates to speed, which is part of cycling efficiency.
Why a high gear can feel hard on the body
Pushing a tall gear slowly asks your muscles and joints to produce a lot of force on each stroke. Some riders find this tiring and uncomfortable in the knees, particularly on long climbs or from a standstill. Others like the feel of a big gear for short, strong efforts. The effect varies from person to person, and it is not a medical judgement.
If you feel pain in your knees or elsewhere, ease off and choose an easier gear. Persistent pain is a reason to talk to a doctor or a qualified bike fitter, because saddle height and other fit details can also play a part. The topic of high cadence pedalling covers the alternative approach of using lighter gears and turning the pedals faster.
When to shift down
A few common situations call for an easier gear:
- Before stopping or slowing at junctions, so you can restart easily. Choosing a starting gear that is low enough lets you get going without wobbling or straining.
- Before a climb, since changing gear under heavy load is harder on the drivetrain. The reasoning is explained in shifting before climbs.
- When you notice your pedalling slowing and your legs feeling heavy.
- When riding into a strong wind or on rough, soft ground such as gravel or grass.
- When riding with a load, a child seat or a trailer, where extra weight makes starting harder.
If you leave shifting until the last moment, you risk crunching gears, dropping a chain or failing to move at all. Easing off the pedal pressure a little as you shift is a common way to help the chain move smoothly. For more on technique, see smooth shifting and the common gear shifting mistakes that beginners make.
How the type of bike changes things
Not every bike shifts the same way. Derailleur bikes expose the chain, and using extreme combinations, such as the largest chainring with the largest cog, can strain it. This is the idea behind cross-chaining. Hub gears hide the mechanism inside the rear wheel and give a smaller number of steps. A single-speed bike has only one ratio, so the choice is made when it is built, which is why the gearing matters so much for single-speed hill tactics.
If you are changing the chainring or cog on any bike, a bike shop can help make sure the parts are compatible and correctly adjusted. Do not guess with parts that affect the chain.
Choosing a ratio that suits you
For everyday riding, a range of gears from easy to moderately tall is usually more useful than one extreme. If you live in a hilly area, a lower overall range may suit you. If you ride mostly flat roads, you may enjoy a taller top gear. If you are not sure, borrow or test-ride different setups, or ask a shop what your bike’s range is and whether it fits your routes.
The practical habit is simple: choose a gear that lets you pedal smoothly at a comfortable pace, and change it early rather than late. Over time you will learn to feel when a gear is too tall or too short, and small, regular shifts will keep the ride calm, efficient and under your control.