I run 18:13 on my Python, as Mark suspected, they're so long and the Geo just makes 13T on the rear, with short(er) cranks feel better.
If you've got 170mm cranks, you can run 12T really.
My mate had an XTP and he ran 18:13 for a bit but then changed back to 12T cos he got longer cranks I think.
If you're quite a small person and it just feels so heavy, like mine did, try a 13T rear (Y)
Or even better, new cranks (Y)