and:

${\mathrm{T}}_{\mathrm{small}}=\left(\frac{{\mathrm{N}}_{\mathrm{small}}}{{\mathrm{N}}_{\mathrm{large}}}\right){\mathrm{T}}_{\mathrm{large}}$

It is sometimes convenient to express gear sizes in terms of the number of teeth on the gear. The ratio of teeth on the large and small gears is identical to the ratio of gear diameters (or gear radii).