My suggestion is to look up how gear ratios are calculated for bikes... there are a few different terms, but a common one is "gear inches," which is the number of inches that you move for one complete revolution of the pedals. Also think about "cadence" which is revolutions per minute of your pedals. There are online calculators that will work these things out for you, along with things like the speed that you can ride for a particular gear combination at a particular cadence.

This is practically pure math, which scares some people, but is not really all that difficult. There's no absolute answer because each rider has their own preferred cadence and top speed. For instance for myself, 30+ mph is unattainable due to wind resistance and my own strength, so I have no use for gearing that can reach that speed.