Correct Speedo Gear

TonyMadrid

Registered User
I am running a 1975 Ford C-4 Tranny with a 3.50 rear end gear ratio and BF Goodrich Radial T/A's (295R50/15). The Tire Diameter is 26.7.

Two questions:

1) What size drive gear should be in the 1975 C-4? A seven tooth or an eight tooth?

2) Assuming the seven tooth drive; what speedo gear would I need. When I do the calculations it results in 18.54. Should I go up to the 19 or down to the 18.
Assuming an eigth tooth drive gear the calculations come out to 18.9.

Thanks for any response you may give,

Tony
 
What model of vehicle is the C4 taken from ? If you know that, I can look up the speedo gear chart on the old microfiche card.

From that, if you end up with something in the middle of two gears when you calculate, then you have to decide if you prefer the speedo to show a bit fast, or a bit slow.


cheers
Ed Nicholson
SCCoO
 
TCI Tranny

I bought the tranny used and the guy I bought it from said it was a rebuilt tranny that came from TCI. The casting mark is really worn and not very legible. I can see the characters D5 then it looks like it might be an O but I can't be sure.

Did Ford use an eight tooth drive gear in 1975. Someone once told me that Ford didnt start using the 8 tooth drive until after 1980 but he wasn't certain.

I have tried to count the teeth by placing my little finger into the speedo gear hole but it is hard to tell. I'm trying to avoid having to tear the tailshaft housing open.
 
From what I can see on my old yellowed, scratched up 1973 - 1979 microfiche card, 1975 C4 should have an 8 tooth drive gear on the output shaft. So in that case, from your gear ratio and tire diameter, you would need approx. 21.5 teeth on the driven gear. The highest tooth count available from Ford is 21, p/n DOOZ-17271-B. That will be as close as you can get with readily available Ford speedo gears.


cheers
Ed N.
 
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