Did you want to use an internally geared hub similar to a Sturmey-Archer 3-speed hub?
If so, you need to know that the axle length on most hubs of that ilk apparently presumes a 120mm rear dropout spacing ...
Regardless, what you probably need to know is that the range of the gear ratio is not particularly wide ... well, it is wide enough for some types of riding BUT may not be for what you want.
So, you would choose the cog size according to the final equivalent that you wanted ... by my reckoning (a long time ago), a
[COLOR= #0000ff]23t[/COLOR] cog equals a[COLOR= #ff0000]
29t [/COLOR]when in engaged in the lowest of the three gear ratios ... so, the ratio change must be about
30% from the middle gear ... and, a 23t cog would be equivalent to a
[COLOR= #ff0000]16t[/COLOR] when the hub is engaged in the highest gear ...
Calculate the range YOU want, accordingly.
- a 20t cog will yield a 14t high equivalent & a 26t low equiavlent ...
- et cetera
OF COURSE, you will want to choose the chainring according to the low ratio you want to use.