Everyone has different requirements for gearing on a folding bike. I'm writing this article, hence we will use my rational. It is important to find a perfect low gear, it's useless if it's too low. While climbing would be impossible if it's too high. Folding bikes are more efficient at climbing due to the small wheels, hence the low gear can be a little higher than a big wheel bike. I have found 22 gear inches to be too low for me. This gear is way to low and essentially unusable. 

I have found I can climb mountains with 28 gear inch low gear, therefore I believe most riders can use 28 gear inches to climb most hills. I understand some customers have swapped to a 25 gear inch low gear, and others have gone to 22. However I think specing bikes with a 28 granny allows hill climbing and leaves enough on the table for a solid high gear. We use 11-34 cassettes on most bike. Therefore the top gear is around 90 gear inches. Which should allow riders to top 20 mph.

Ultimately it is important to find the perfect low gear, that will define your highest gear ( based on the cassette or internal hub)