Hello-
I also just scored a treadmill motor like that. Ratings about the same and 24Amps.
I'll probably just make a nice 100Watt Pedal excercise generator with it for my bedroom.
I've made a couple windmills using them-- and they are a fun starter project.
That is a 'Fast' treadmill motor like the majority of them-- so If it were me, I would make a 6 blade pvc 3 foot diameter mill out of it. (6 blades to get over the cogging starts. It would take a hurricane with only 3 blades)
I think you will need the smaller 3 ft diameter in order to get the higher speed (Without horrible noise like a fast 4 footer) for that motor.
It could just be me, but I have Never been able to make a 3 blade mill that DON'T Make that horrible wirling noise (NO furling mechanism) in giant wind gusts. I hate noisy mills.
Even though it will do less MAX power out in big (rare) wind gusts than a 4 footer, it should generate way MORE OFTEN as a 3' mill than a 4' mill I believe.
Might try Making the blades out of (typical 4" diam. Plumbing) PVC pipe similar to this one I made here.
http://www.fieldlines.com/story/2009/2/7/194335/8871
I think these 6 blades do WAY better in turbulant wind than 3 blades, and most important of all the 6 blade made NO NOISE in giant wind gusts with NO furling mechanism. During some 50mph wind gusts the blades must of been going over 3000rpm without making a sound. I was standing outside, but at a safe location-- and it was quite surprizing to me.
Put it at a safe Height in the better winds for best Watt hour production -- and put a piece of aluminum or sheet metal around the open enclosures on the motor with a big hose clamp and some caulk to keep the rain out-- and see what it does. (Might do up to a 100 watts in big wind gusts. The 32" I made only did upto 50 watts since the alternator I made was high resistance and wimpy, but a very fun first project!)
Hope I help.