Whats your Brompton gear ratio?

As of now, late 2014, Bromptons (from the factory) come in the following gearing configurations:

Brompton 54T chain ring

Single speed
12 tooth rear sprocket
54 tooth front chain ring

Two speed
12T/16T rear sprokets
54 tooth front chain ring

Three speed (Brompton standard ratio)
13T rear sprocket
50 tooth front chain ring

Six speed (Brompton wide ratio)
13T/16T rear sprockets
50 tooth front chain ring

They are all available in a reduced gearing version, by replacing the front chainring with a smaller one - giving approx 10% reduction (so 54 tooth is replaced with 50 tooth on the 1 and 2 speed models and the 50 tooth is replaced with 44 tooth on the 3 and 6 speed models)

However - there are more variations available - such as reducing from 54T to 44T on the 1 or 2 speed Brompton, or changing from 12T to 13T or 16T on the single speed (for example)

Brompton Gear Calculator

We are sometimes asked for the exact ratios (and to be honest we are not that scientific)

This website seems to do just the job

It allows you to select standard configurations and gives the gear inches that you will get.

It even takes into account the ratio change based on different tyres (unfortunately it's slightly out of date - so Schwalbe Marathons still show as 16 x 1 3/8 " where as in fact they are now the same size as Marathon Plus at 16 x 1.35)

But it will give a good idea of what would happen if you changed the hub and or chainring.

Have fun

Related resources:

Brompton gear calculator
Brompton spares and accessories from Brilliant Bikes