first do u have a gear to put in and be able to run it??? if no then there are a few web sites (summit/jegs) that have a calculation table to fill in the blanks and it's give u the driven gear teeth. now if u do have a gear to use, put it in and drive it with a GPS going and looking at the speedo. 60 is a good mph. then divide the big number into the small to get a percentage. then multiply that % to the driven gear teeth count and u'll get the number of teeth to add or subtract to that one. i did the last part on my fairlane and it worked very well.. the web sites were a royal pain.