Gear Ratio and MPH

If your speedometer reads too low, use a speedometer drive gear with fewer teeth. If your speedometer reads high, you need a drive gear with more teeth. Do this in one-tooth increments. A drive gear with 18 teeth is going to turn faster (and read faster) than a drive gear with 21 teeth. If your speedometer is reading 10 mph too high and you have an 18-tooth drive gear, you need a 20-tooth drive gear. The highest number of teeth available on a Ford speedometer drive gear is 21. After that, you must run a reducer that was common to Fords with 3.89:1, 3.91:1, 4.11:1, and 4.30:1 rearend gears. Taller tires will make the speedometer read lower. Shorter tires will make it read higher. Even tire inflation can affect speedometer reading.

Manual Transmission

16-teeth-wine (C0DZ-A) 17-teeth-white (C3DZ-C) 18-teeth-yellow (C0DD-B)
     
19-teeth-pink (C0DZ-B) 20-teeth-black (C1DZ-A) 21-teeth-red (C4OZ-A)
     

Automatic Transmission

16-teeth-blue (D0AZ-A) 17-teeth-green (C7SZ-A) A) 18-teeth-gray (C7SZ-B)
     
19-teeth-tan (C7VY-A) 20-teeth-orange (C8SZ-B) 21-teeth-purple (D0OZ-B)
RPM= MPH x Trans Ratio (in high gear) x Rear end Ratio x 336
Tire height
13 inch tire size 14 inch tire size 15 inch tire size

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