I'm also trying to figure out the right gearing before I make the conversion to Fixed. I've tried several on my bike (while still geared), and I can see many working fine. However, the only the smaller (42t) chainring can be removed from the cranks. So I'm left with the 52t. The calculations work out best for a 20 tooth cog (52/20 = 2.6 x 27.559 = 71.65, or 19 52/19 = 2.7368 x 27.559 = 75.42) Anything smaller is too long.

Can anyone think of problems this would cause? I'm going now to try to take the cranks and 42t off of my roommate's junk bike.

Thanks for your help.