The first thing to know is how many revolutions the tires make in one mile. To calculate this, measure the diameter of your tires. 26" is about average for F60-15's or 245-60-15's. Multiply this measurement (26") by pi (3.1416). This will give you the circumference of the tire. Next, divide one mile in inches (63360) by the circumference of tire (63360 divided by (26 x pi)=776). This equals the revs per mile of the tire. Now multiply this number by the rear axle ratio to give you the number of drive shaft revs per mile (776 x 3.91= 3033). Next, divide this number by 1000 (3033 divided by 1000 = 3.033). Now multiply this number by the number of teeth of the drive gear in the trans (count 'em if you have to!) In this case we will assume a drive gear with 6 teeth and that means the we need to use a driven gear with 18 teeth (3.033 x 6 = 18.198), as we need to pick the available gear closest to the answer. 18 is the best choice, which yields a +1.1% (actual gear teeth (18)/calculated gear teeth (18.198) = 1.011) error which means your odometer unit indicates 1.1% greater than actual or 10.11 for an actual 10.0 miles. +/- 3.75% is the accepted tolerance from the factory. If you are using the Ford adapter/reducer (reducer was factory equipped on cars with 3.91, 4.11 or 4.30 rear axle ratios), multiply the number by 0.75 (3.033 x 0.75=2.274). This becomes is the driven/drive ratio you need.
Now, not all C6s have the drive gear machined onto the output shaft. Some are plastic slip on and color coded. The options are 6 (pink), 7 (steel or black), and 8 (brown) teeth. If you cannot count the teeth but do know the original tire size, driven gear tooth count and rear axle ratio, you can back it out using the above formula.
Driven gear options from Ford are: 16 (burgandy or blue), 17 (green), 18 (gold/yellow or grey), 19 (tan), 20 (black or orange) and 21 (red). Steeda (
http://www.steeda.com) makes a 23 tooth driven gear for a Ford auto trans. The thin teeth do have a finite life, however
speedo cable gear?
For us who are math challenged, here is a calculator that will get you the speed you are going. Now the number of teeth to get you right will be solved Twister's way. 
