Gear inches are part of an endearingly archaic system that works out the size that the bike's driving wheel would be if only the pedals were connected directly to it – as on a child's tricycle or a penny-farthing high wheeler.

The dinky red kiddy trike has a front wheel of fourteen inches—hence, 14 gear inches.

By contrast, the ever-so-macho penny-farthing has a driving wheel five feet high—60 gear inches.

A circus clown's unicycle might have a 20-inch wheel—20 gear inches.

Gear Inch Formula…

tyre diameter x chainring / cassette = gear inches

Example A: 48-tooth chainset and 24-tooth rear sprocket:

• 26in x 48 / 24 = 52 gear inches
• diameter x chainring / rear sprocket = size of penny-farthing's wheel

Example B: 48-tooth chainset and 16-tooth rear sprocket:

• 26in x 48 / 16 = 78 gear inches
• diameter x chainring / rear sprocket = size of penny-farthing's wheel