Speedometer Calibration
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Speedometer Calibration
Hey guys,
I need help figuring out how many drive gears my C4 Automatic transmission has. I have 235 75 15 tires with a 3.07 rear differential ratio and an 18 count driven gear tooth. I just had my rear differential rebuilt so now the gears went from 2.72 to 3.07. How many teeth do i need on my driven gear? Also, how many teeth does the drive gear from the transmission have on it for a C4?
I need help figuring out how many drive gears my C4 Automatic transmission has. I have 235 75 15 tires with a 3.07 rear differential ratio and an 18 count driven gear tooth. I just had my rear differential rebuilt so now the gears went from 2.72 to 3.07. How many teeth do i need on my driven gear? Also, how many teeth does the drive gear from the transmission have on it for a C4?
Fleet Mechanic
Joined: Feb 2006
Posts: 1,393
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From: MA
Easiest way I found
Have a cell phone with GPS? download the free speedometer app.
Get the truck up to 60 MPH according to the GPS.. Read the speedometer and record that number.
See below for math
I just did my F100 with this method.
Then count how many teeth your speedometer cable gear currently has (18 you stated).
Have a cell phone with GPS? download the free speedometer app.
Get the truck up to 60 MPH according to the GPS.. Read the speedometer and record that number.
See below for math
I just did my F100 with this method.
Then count how many teeth your speedometer cable gear currently has (18 you stated).
Fleet Mechanic
Joined: Feb 2006
Posts: 1,393
Likes: 5
From: MA
62 MPH x 18 / 75 = 14.88. Round up to 15 teeth. I don't think those even exist. You'll be forced to change the drive gear on the output shaft of the transmission so you can utilize an existing gear.
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Fleet Mechanic
Joined: Feb 2006
Posts: 1,393
Likes: 5
From: MA
Ok. I'm an idiot and I apologize.
Speedometer Gear Color Code Specification Chart With Formulas, And Ford Part Numbers. - Ford Explorer Ranger Enthusiasts "Serious Explorations"®
This is the formula I used. I typed it wrong. You are correct.
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.
SO..
X = (18 X 75) / 62 = 21.77
Your theory is correct, my math failed, lol.
Speedometer Gear Color Code Specification Chart With Formulas, And Ford Part Numbers. - Ford Explorer Ranger Enthusiasts "Serious Explorations"®
This is the formula I used. I typed it wrong. You are correct.
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.
SO..
X = (18 X 75) / 62 = 21.77
Your theory is correct, my math failed, lol.
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