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?

 

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).

 

Ok I live in canada so when am going 100 KPH real my speedo says 120? what do i do then?

 

Or when going 62 MPH real my speedo says 75 MPH

 

I did the math (with rounding) It says i need 15 tooth count? Is this right?

 

But that doesnt make sense because less teeth=more rpms and more teeth= less rpms of speedo?

 

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.

 

this is soo confusing i dont even know what to say...

 

would i have to lower or raise the drive gear teeth ?

 

but was the less teeth=more rpms and more teeth= less rpms of speedo theory correct?

 

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.

 

so i need 21 or 22 count teeth? Thanks soo much though.

 

Kewel, Im in the same boat with my 79. Keeping up with traffic on I-10. I know I'm going at least 70. Speedo shows 55mph, I really need to change my gear.