A member had asked some time ago about installing a plugin for Illustrator that would help them generate gears for a project they were working on. I suggested they take a look at geargenerator.com as it is a very powerful, free, and parametric solution for creating accurate gear ratios. It can visualize them in motion and export them to DXF or SVG to further edit them. The settings are also encoded into the URL so they are easy to share.
You can see an example of some of its features here: