C&V fixie in progress...
 

I've started to clean out the pile of crap that's accumulated in the barn over the winter, starting with the Gazelle-made Grand Prix I posted in my dump finds. With the dent in the top tube, the only way I'm going to sweeten the deal enough to flip it is to make it a fixie and market it in Portland (ME). It has a flip-flop rear hub, so I guess all I need is a track cog and a lock ring. Question is:what size? I'll use the 40t small ring on the cranK.
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I'd put a 15 on...I ride a 39/15 on my fixed gear. A lot of people use higher ratios, but I really don't think it's good for your knees in the city where you're stopping and starting all the time. Put a low ratio on it and let someone change it if they want to.

I would go even lower with a 16 tooth cog. Better to gear it too low than too high, especially if you're not including a brake in the deal.

I will second the 16t cog. It makes a very nice rideable gear.

I run a 46/17 which would be closest to the 40/15. I use it to commute, there are a couple of hills but nothing too big.