Speedo Gears

I'm assuming that the tire Grim measured is a belted tire. I got the 700/13 and 205/70r/13 diameters (which both have a 24.3 dia.) from the tire manufactures sizing charts. I assume my car has the standard 2.93 ratio rear end. These sizes call for a 19 tooth gear. My tires have a 23.2 dia. It looks like the lower the ratio the lower number of teeth required. The highest number of teeth is 20. I would need at best a 21 tooth gear which doesn't exist.