The primary function of gears are to enable us to maintain a comfortable pedaling speed (cadence) regardless of the gradient or terrain. 

A high gear, sometimes referred to as a ‘big gear,’ is optimal when descending or riding at high speeds. The highest or biggest gear on a bicycle is achieved by combining the largest front chainring size with the smallest rear cog or sprocket. Vice versa, combining the smallest front chainring size with the largest rear sprocket size results in the lowest available gear, which will help you keep your desired cadence when the road points up.

Again, shifting is about pedaling efficiency. Having a much broader choice of gears for a given situation will allow you to apply torque smoothly around the pedal stroke. I recommend having the greatest gear ratio possible for ESR. I’ll be riding a compact crankset 50/32 with an 11–30 tooth cassette on my bike.

Since we are all proficient drivers, I like to use the car analogy to bring home a point. Just like a car, bicycles benefit from a low gear to accelerate from a standstill, or to climb a steep hill, and at the other end of the scale, a high gear helps you to achieve high speeds without over-revving.

Continuing with the car example, using too low a gear at high speed would result in high fuel consumption. The same is true of your body pedaling a bike. More gears means more scope to your pedaling technique by fine-tuning your cadence to suit the gradient or terrain, often resulting in a lower energy cost.