rather than % you can do your own ratio math .. hub cogs 13,14 or 15t factory chain rings , 44,50, 54t..
the recent 5 arm spider crank uses a 130 pcd chain ring so that 39t after market swap can be done. or any other 130 type. 40, 45,46 ,47, etc..
http://sheldonbrown.com/english-3.html
double cranksets can & have been fitted.. I tossed in a Big investment, a Swiss Made Mountain Drive 2 speed Crank .
2 versions of Florian Schlumpfs planetary 2 speed cranks are often used on Bromptons.
Mountain is a /2.5 reduction gear ala 50/20 .. the Speed Drive is an 1.65 x overdrive a 34t acts like a 56t ..
MD uses the 3 hub hears twice .. i have a 15:54t
the 6 factory options, via
http://sheldonbrown.com/gears/internal.html
[TABLE]
[TR]
[TH="bgcolor: yellow"]
0.75 (Low)[/TH]
[TH="bgcolor: yellow"]
1.0[/TH]
[TH="bgcolor: yellow"]
1.33 (High)[/TH]
[/TR]
[TR]
[TD][TABLE]
[TR]
[TD][/TD]
[TH="bgcolor: yellow"]
54[/TH]
[TD="align: center"]
8.0 %[/TD]
[TH="bgcolor: yellow"]
50[/TH]
[TD="align: center"]
13.6 %[/TD]
[TH="bgcolor: yellow"]
44[/TH]
[/TR]
[TR]
[TH="bgcolor: yellow"]
13[/TH]
[TH="bgcolor: white"]50.1[/TH]
[TD][/TD]
[TH="bgcolor: white"]46.4[/TH]
[TD][/TD]
[TH="bgcolor: white"]40.8[/TH]
[/TR]
[TR]
[TD="align: center"]
7.7 %[/TD]
[/TR]
[TR]
[TH="bgcolor: yellow"]
14[/TH]
[TH="bgcolor: white"]46.5[/TH]
[TD][/TD]
[TH="bgcolor: white"]43.0[/TH]
[TD][/TD]
[TH="bgcolor: white"]37.9[/TH]
[/TR]
[TR]
[TD="align: center"]
7.1 %[/TD]
[/TR]
[TR]
[TH="bgcolor: yellow"]
15[/TH]
[TH="bgcolor: white"]43.4[/TH]
[TD][/TD]
[TH="bgcolor: white"]40.2[/TH]
[TD][/TD]
[TH="bgcolor: white"]35.4[/TH]
[/TR]
[/TABLE]
[/TD]
[TD][TABLE]
[TR]
[TD][/TD]
[TH="bgcolor: yellow"]
54[/TH]
[TD="align: center"]
8.0 %[/TD]
[TH="bgcolor: yellow"]
50[/TH]
[TD="align: center"]
13.6 %[/TD]
[TH="bgcolor: yellow"]
44[/TH]
[/TR]
[TR]
[TH="bgcolor: yellow"]
13[/TH]
[TH="bgcolor: white"]66.8[/TH]
[TD][/TD]
[TH="bgcolor: white"]61.8[/TH]
[TD][/TD]
[TH="bgcolor: white"]54.4[/TH]
[/TR]
[TR]
[TD="align: center"]
7.7 %[/TD]
[/TR]
[TR]
[TH="bgcolor: yellow"]
14[/TH]
[TH="bgcolor: white"]62.0[/TH]
[TD][/TD]
[TH="bgcolor: white"]57.4[/TH]
[TD][/TD]
[TH="bgcolor: white"]50.5[/TH]
[/TR]
[TR]
[TD="align: center"]
7.1 %[/TD]
[/TR]
[TR]
[TH="bgcolor: yellow"]
15[/TH]
[TH="bgcolor: white"]57.9[/TH]
[TD][/TD]
[TH="bgcolor: white"]53.6[/TH]
[TD][/TD]
[TH="bgcolor: white"]47.1[/TH]
[/TR]
[/TABLE]
[/TD]
[TD][TABLE]
[TR]
[TD][/TD]
[TH="bgcolor: yellow"]
54[/TH]
[TD="align: center"]
8.0 %[/TD]
[TH="bgcolor: yellow"]
50[/TH]
[TD="align: center"]
13.6 %[/TD]
[TH="bgcolor: yellow"]
44[/TH]
[/TR]
[TR]
[TH="bgcolor: yellow"]
13[/TH]
[TH="bgcolor: white"]89.0[/TH]
[TD][/TD]
[TH="bgcolor: white"]82.4[/TH]
[TD][/TD]
[TH="bgcolor: white"]72.5[/TH]
[/TR]
[TR]
[TD="align: center"]
7.7 %[/TD]
[/TR]
[TR]
[TH="bgcolor: yellow"]
14[/TH]
[TH="bgcolor: white"]82.6[/TH]
[TD][/TD]
[TH="bgcolor: white"]76.5[/TH]
[TD][/TD]
[TH="bgcolor: white"]67.3[/TH]
[/TR]
[TR]
[TD="align: center"]
7.1 %[/TD]
[/TR]
[TR]
[TH="bgcolor: yellow"]
15[/TH]
[TH="bgcolor: white"]77.1[/TH]
[TD][/TD]
[TH="bgcolor: white"]71.4[/TH]
[TD][/TD]
[TH="bgcolor: white"]62.8[/TH]
[/TR]
[/TABLE]
[/TD]
[/TR]
[/TABLE]
but as you say :
I'm hoping to get a Brompton soon,
perhaps your needs will be better served with the BWR3x2 6 speed, 12-16t same chainring choices up front
BWR, a wider 3 speed than BSR, takes 2 cogs ..leave it in a 12 for faster, 16 for hills ,
maybe even retrofit.. a 36-44 double crank .
[Todd, the PDX Brompton shop owner, Tours on His.. like that no FD so still just 2 levers.]