13-24, actually.

I have the top gear for cruising and two gears for climbing/headwinds, and if I spin out I can coast down hills. I like this better than powering downhill and walking up. I'm not the only one who sets three-speeds up this way.

Don't worry about efficiency in the gear ratios - a broken-in and properly lubricated three-speed hub is amazingly efficient - on average, better than a high end derailleur gear train, and arguably the most efficient multi-gear bicycle drivetrain available.

On my tikit, with the same wheel size as your Brompton, I use a SRAM iMotion3 hub, 53x16, giving 39/53/73g.i.