The S.I.R. is made to be run with whatever gear setup you choose. It has an eccentric bottom bracket to pick up chain slack if you run at is a single speed and it also has a replaceable rear dropout with an extra dropout provided, one for a derailleur and the other without the derailleur hanger for a nice and clean single speed look.
The Reynolds 853 steel tubing feels great, and the colour is awesome. As it turns out it is easy being green.