The ratios you're talking about are the ring & pinion ratios. Porsche usually refers to them by the # of teeth on the pinion gear and ring gear respectively....... ie 8:31. 8 is the number of teeth on the pinion gear and 31 is the number of teeth on the ring gear. For most cases (in all vehicles), the pinion gear will have less teeth than the ring. A 915 came in either a 7.31, which is a 4.43 and a 8:31 is a 3.88. This is calculated by simply dividing the ring gear teeth by the pinion teeth. ie. 8:31..... 31/8= 3.875 or 3.88. Also try
www.phii.net They have a list of pretty much all of the gears available for the 915. Weddle should have what you need for the VW stuff. Yeah... my trans is gonna cost me. That's why I'm running it stock for a while. Changing every gear but 1st. ZF's can be rebuilt. ZF's are becoming harder and harder to find. But there are companies that make the same style diff. Quaife's in the US cost around $1100. for the VW. Not including the labor. Good Luck and keep us posted on the VW ZF's. Thanks!