Christophe De Vocht has developed a Drive Calculator program to work out drive ratios for all of Serpents current, and some older, models. The user can input all the pulley sizes as well as the tyre diameters for it to work out the ratio, it will also suggest a tyre size to get a user determined front to rear drive ratio too.

The drive calculator is available to download on the 960, 835 and 720 download pages. Check it out.