Yes, perhaps "maintain" was not the best choice of words. If we want to cover distance, on flat roads with no wind (is there such a thing?) an average of around 18 mph for an hour plus would be more likely. But for those with a better engine, the gearing would allow 24mph with a cadence of 90. Wind at back or slightly downhill averaging 24mph is very achievable. I have gone over 30 downhill on the Brompton but was not completely comfortable going that fast on it.

The upright position on the Brompton compared to my road bike means I work harder to go the same speed. But usually I am not concerned with getting from A to B as quickly as possible, when traveling.

I guess my main point is that I find the gearing to be plenty high enough and believe it best for the traveller to be more concerned with the low gearing, unless one is a particularly strong rider. The overall gearing range of the 6 speed has worked out extremely well for us.