How can I controll the speed of motor using gears? Can anyone tell me how to use gear wheels to modify the speed of the motor in the Power Functions set 8293? 

I know I could use the remote control unit, but I need to have a fixed slow speed on a static unit which will power a non-LEGO moving toy. I need a slow turning axle.
Sariel's tutorial is one of the better ones, and his whole web site is worth while. The Eurobricks Technic forum also has a list of tutorials thread that you might find helpful.

Sariel starts very simple and ends up with a discussion of what makes different variants on some of the Lego gears effective...