To answer your question--you need to order the complete set from mazda with a matched ring and pinion. You do not want to try this if you have to ask the question. It takes some specialized equipment and fixtures to do it right. If you have never set up a rear gear before I would have that done by someone qualified or you will have some expensive scrap metal after the first race. Not trying to be harsh, just don't want you to make an expensive mistake.
[/b]
Thanks a lot for your respond and advice. Don't worry because I won't be doing the job myself. I have the mechanic and the machine shop with the necesary equipment to do the job nicely. I just want to know what is the necessary ring and pinion combo to do the conversion for at least a 4.56 or lower ratio. You said I need to buy both ring and pinion set from a first gen RX-7. Do you know from what year and model ?? My car is street driven but I run actively in time trials events and would love to take advantage from the benefits of lower gears.