[roofer]

Im going to try a single speed what size freewheel should I start with.

 

[slocaus]

Rule of thumb starting point is a 2:1, such as 36:18 or 32:16, etc. It depends on what you have to climb. Best quote I have seen here is from Shiggy : "Gear for the climbs and learn to spin" (or something like that).

On my 26 SS I had to use a 34:20 and my 29er SS has a 32:22 to give the same inch gear. I'm in central CA where we have some steep climbs (plus I'm carrying a few pounds which the SS is melting away - out of every pore on my body).

Single speed freewheels are cheap, so you can have 2 or 3 without breaking the bank.

 

[anotherbrian]

Im going to try a single speed what size freewheel should I start with.

Rather than follow the 2:1 rule of thumb, just ride your current bike in one gear over the terrain you normally would, and find one that works. You'll need to figure out your own style for climbing (a reasonably fast cadence or a very slow grind) and its effect on your speed (spin out at 9mph or something more reasonable).