The files available on the Reprap wiki don't work properly with the belts I got from McMaster Carr. Mine say "Gates Powergrip made in USA" on the side and they look different, visually, from the ones I've seen in pictures of Mendels.
The build file clearly claims that .2" pitch belts will work in place of the 5mm belts it normally uses, but the timing pulleys I printed out weren't anywhere close to 5 teeth per inch. One was 6/in and one was 5.5/in. Since they weren't even the same pitch I figured maybe the files were improperly scaled or something. So, I resized them until they were the correct pitch, but the teeth are too thin/far apart and leave one or two steps of play.
I'm trying to generate a new set of pulleys based on the dimensions of the belt I have. I was going to do the math myself, but Erik already did it...so yay! thingiverse.com/thing:1362