How do I ..? Know what gear I'm in

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Written by: Tim Costello, 5 February 2009
Last updated: 5 February 2009

Introduction

What gear?

If you drive a car it's fairly easy to know what gear you are in. It's either reverse, first, second, third, fourth or fifth, or sometimes nowadays sixth. The concept is easy. The higher the number the faster you can go.

With bikes it's a bit more complex than that. Not only are there more gears to contend with, up to three chainrings on the front and eleven sprockets on the back, but which is bigger 53x21 or 39x15?

The purpose of this How do I? guide is to explain how you can compare different gear ratios.

What does he mean 53x21?

It is the normal convention when referring to a gear to give the number of teeth on the chainring first followed by the number of teeth on the sprocket. For example 53x21 means 53 teeth on the front chainring and 21 teeth on the rear sprocket.

The ratio of the number of teeth on the chainring to the number of teeth on the sprocket and the circumference of the wheel determine the distance you will travel for every revoloution of the cranks. The bigger the ratio the further you will travel.

Comparing ratios

Traditionally, in Britain we have compared different gears by reference to inches (how quaint). For example a 70 inch gear is smaller than an 80 inch gear, i.e. I will travel further for each revoloution of an 80 inch gear than a 70 inch gear.

The inches used to have some meaning, but nowadays they are just a mechanism for comparing different gear ratios. Over the water, "on the continent", they sneakily (as you'd expect) specify how far in metres you will travel in a given gear ratio for one revoloution.

In a spirit of Little Englandness we'll stick to inches, if only because no one over here will have a clue what you're talking about otherwise.

To calculate the number of inches for a gear ratio:

(Number of teeth on chainring / Number of teeth on sprocket ) x 27

This assumes you have a 27 inch (or 700c) wheel.

Thus:

53x21 = (53 / 21) x 27

53x21 = 68 inches

39x15 = (39 / 15) x 27

39x15 = 70 inches

Gear ratio table

The following gear ratio table provides ratios for the most popular chainset combinations, i.e. 53/39 and 50/34 (compact), plus my favoured chainset for cyclo-cross 46/36.