Determining millimeter pulley sizes

Have a dozen or so pulleys around and only 3 have the 13.9 or 14mm stamped on them. I may even have some 8mm old stuff mixed in, so I need to know how to measure or just look at them to tell. An what's the diff between a 8mm and 8mm GT...pulley? Thanks..Warren

 

Go to the rcdengineering website. Look up belts. Open the various sizes. The belt number looks like center to center of teeth. .551 = 14mm. You should see the difference between HTD and GT also. If I am wrong somebody else help ???