Brompton G Line Sprockets

The Brompton G Line uses a Shimano Nexus sprocket 

As standard it comes with a 20 tooth sprocket

If you want to reduce the gearing slightly - a 22T sprocket will fit

When riding at a cadence of 70 rpm in the lowest gear with a 20T sprocket you will be going 9kmh
If you increase the sprocket to 22T and pedal at a cadence of 70 rpm your speed would drop to 8.1kmh

Note on increasing longevity of your sprocket:

Keep it clean - as dirt accumulates on the sprocket and is mashed by the chain, it will wear out both the chain and the sprocket. Keeping it clean will extend the life of both!