From TCCOA:
If you have an automatic with a 7-tooth output shaft (used on 94-97 Thunderbirds/Cougars, 93-98 Lincoln Mark VIII w/4R70W):
3.08 - 17 tooth, Green (C7SZ-17271-A)
3.27 - 18 tooth, Gray (C7SZ-17271-B)
3.55 - 20 tooth, Orange (C8SZ-17271-B)
3.73 - 21 tooth, Purple (D0OZ-17271-B)
If you have automatic with an 8-tooth output shaft (used on 89-93 Thunderbirds/Cougars w/AOD and all other 4R70Ws from other vehicles):
2.73 - 17 tooth, Green (C7SZ-17271-A)
3.08 - 19 tooth, Tan (C7VY-17271-A) for 225/60R16 ; 20 tooth, Orange (C8SZ-17271-B) for 215/70R15
3.27 - 21 tooth, Purple (D0OZ-17271-B)
If you have a manual with an 8-tooth output shaft (used on 89-95 Thunderbirds/Cougars w/M5R2):
2.73 - 17 tooth, White (C3DZ-17271-C)
3.08 - 19 tooth, Pink (C0DZ-17271-B) for 225/60R16 ; 20 tooth, Black (C1DZ-17271-A) for 215/70R15
3.27 - 21 tooth, Red (C4OZ-17271-A)
The above is valid for both stock tire sizes of 215/70R15 and 225/60R16. Bigger tires will require gears with less teeth and vice versa.
If you require a speedo gear with less teeth than 16 or more teeth than 21, you need a device like the SpeedCal.
To calculate what speedo gear you might need for your custom-sized tire, first look at the specs from the manufacturer that list the revolutions per mile. Then do the following calculation:
(Number of Output Shaft Teeth) * (Rear Gear Ratio) * (Tire Revolutions per Mile) and divide this by 1,000, then round to the nearest whole number.
As an example, take a 245/55R17 Goodyear g-Force T/A listed at 766 revs/mile on a stock 1996 MN12 with 3.27 rear gears.
7 * 3.27 * 766 = 1753 ÷ 1,000 = 17.53, which rounded is 18.
