If you are looking for lowest possible gears, mountain-drive is your ultimate choice.

A chainring with 50 teeth -> equals a chainring with 20 teeth in low gear mode.
A chainring with 40 teeth -> equals a chainring with 16 teeth in low gear mode.
Smallest possible chainring is 34t. -> equals a chainring with 14 teeth in low gear mode.
For very special applications there is a chainring with 27t. available -> equals a chainring with 11 teeth in low gear mode.

If you use a larger than the original chainring, you get both higher and lower gears.

Biggest chainring available from Schlumpf: 75 t. -> equals a chainring with 30 teeth in low gear mode.

Black bars in the diagram on the right show additional gears in combination with mountain-drive and a "good old" 3-speed-hub -> you get nearly the gear range of a mountain bike!

Please find a gear range calculator (for meters per revolution) here:
-> in combination with derailleurs
-> in combination with internal hubs

Please find a gear range calculator (for gear inches) here:
-> in combination with derailleurs
-> in combination with internal hubs