- Joined
- November 11, 2005
- Messages
- 56,691
- Reaction score
- 1,186
- City, State
- Brooklyn, NY
- Year, Model & Trim Level
- 88 89 93 95 96 Aerostars
Changing The Speedometer Gearing.
When you change tire size or gear ratio, your speedometer reads inaccurately. To fix this you have to change the speedometer gear (change the number of teeth). First you need to find out the current speedometer gear's tooth count. You have to remove the speedometer cable from the transmission (instructions below). The gears are color coded. Here is the chart with Ford part numbers:
Manual Transmissions:
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)
Auto Transmissions:
16-teeth-blue (D0AZ-A)
17-teeth-green (C7SZ-A)
18-teeth-gray (C7SZ-B)
19-teeth-tan (C7VY-A)
20-teeth-orange (C8SZ-B)
21-teeth-purple (D0OZ-B)
Then Use This Formula:
New Teeth = (Current Teeth x Speedo Reading)/Actual Speed.
Ford Motorsports Also Uses This Formula:
Driven Gear Teeth = (Drive Gear Teeth x Axle Ratio x Tire Rev. Per Mile) / 1000.
FourWheeler Magazine uses:
Actual MPH= (New Tire Diameter x Indicated MPH) / Old Tire Diameter.
The first two formulas will tell you how many teeth your speedometer gear needs. You can obtain the new gear from your local Ford dealer. The swap will provide you with the approximate correct MPH readings.
The speedometer gear is attached to the black cable going into your transmission's tailhousing. 4wd vehicles - the speedo sensor is located near the rear output flange of the transfer case. 95, and above did not use a cable. Digital dash doesn't use a cable either. 98, and above started to use the sensor on the rear differential. This is also used for the ABS. There is a single bolt, and retaining ring holding it in. Once you remove the bolt, and ring, the gear, and cable will slide right out. The gear is attached to the cable with a small clip which could be removed with a small screwdriver.
Note: My 89 Aerostar has digital dash, and doesn't have a cable, but has the VSS on the extension housing. My 93 Aerostar with analog dash does not have a cable, or a VSS on the extension housing. It only has the sensor on the rear differential.
2WD vehicles:
Refer to the chart for automatic transmissions.
4WD vehicles:
Refer to the chart for manual transmissions. 4WD vehicles use the same gear as a manual transmission regardless if the transmission is a manual, or an automatic.
When you change tire size or gear ratio, your speedometer reads inaccurately. To fix this you have to change the speedometer gear (change the number of teeth). First you need to find out the current speedometer gear's tooth count. You have to remove the speedometer cable from the transmission (instructions below). The gears are color coded. Here is the chart with Ford part numbers:
Manual Transmissions:
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)
Auto Transmissions:
16-teeth-blue (D0AZ-A)
17-teeth-green (C7SZ-A)
18-teeth-gray (C7SZ-B)
19-teeth-tan (C7VY-A)
20-teeth-orange (C8SZ-B)
21-teeth-purple (D0OZ-B)
Then Use This Formula:
New Teeth = (Current Teeth x Speedo Reading)/Actual Speed.
Ford Motorsports Also Uses This Formula:
Driven Gear Teeth = (Drive Gear Teeth x Axle Ratio x Tire Rev. Per Mile) / 1000.
FourWheeler Magazine uses:
Actual MPH= (New Tire Diameter x Indicated MPH) / Old Tire Diameter.
The first two formulas will tell you how many teeth your speedometer gear needs. You can obtain the new gear from your local Ford dealer. The swap will provide you with the approximate correct MPH readings.
The speedometer gear is attached to the black cable going into your transmission's tailhousing. 4wd vehicles - the speedo sensor is located near the rear output flange of the transfer case. 95, and above did not use a cable. Digital dash doesn't use a cable either. 98, and above started to use the sensor on the rear differential. This is also used for the ABS. There is a single bolt, and retaining ring holding it in. Once you remove the bolt, and ring, the gear, and cable will slide right out. The gear is attached to the cable with a small clip which could be removed with a small screwdriver.
Note: My 89 Aerostar has digital dash, and doesn't have a cable, but has the VSS on the extension housing. My 93 Aerostar with analog dash does not have a cable, or a VSS on the extension housing. It only has the sensor on the rear differential.
2WD vehicles:
Refer to the chart for automatic transmissions.
4WD vehicles:
Refer to the chart for manual transmissions. 4WD vehicles use the same gear as a manual transmission regardless if the transmission is a manual, or an automatic.