Speedo Gear Info Needed

TonyMadrid · #1 (**permalink**) 11-13-2003, 05:24 AM

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 eight tooth drive gear the calculations come out to 18.9.

Thanks for any response you may give,

Tony

Karl Bebout · #2 (**permalink**) 11-13-2003, 06:21 AM

Tony, I did an internet search for something like "speedometer gear calibration" and came up with several sites that gave explicit information about which combination of drive and driven speedo gears were needed for various rear end gears and tire size. Saved me lots of trial and error and I was able to get the right gear from a transmission shop for like $5. Much cheaper than a dealership. The gear was used but in good shape and I've had no problem. Karlos

Ron61 · #3 (**permalink**) 11-13-2003, 07:48 AM

Tony,

Ford says that each tooth on a driven gear will equate to about a 4 MPH change. You can be much closer depending on your setup but may be off at some ranges. If it were me, I would go with one that reads just a little fast, IE maybe 1 MPH if I could to avoid the speeding tickets for driving 2 MPH over the limit. And yes, they do give them out for that, at least here. I got one going to work for doing 47 in a 45 MPH zone at 3 AM with no traffic except me and the cop. My speedometer was reading 45 but it still cost me $55 dollars.

Ron

TonyMadrid · #4 (**permalink**) 11-13-2003, 08:08 AM

Thanks for the info but what I am trying to find out without having to tear down the tailshaft is whether the drive gear is a seven or an eight tooth gear. I have tried counting the teeth by placing my little finger in through the speedo gear opening but it is hard to tell.

Someone once told me that Ford didn't start using an eight tooth drive gear until 1980 but he wasn't certain.

It is not a major concern right now as I can try an 18 tooth driven gear and measure my speed against another vehicle's speed (son's car) then work it up or down from there.

But if anyone knows for certain if the drive gear is an eight or seven tooth gear it would make things simpler.

Ron61 · #5 (**permalink**) 11-13-2003, 09:50 AM

Tony,

This isn't going to cheer you up much but I called a man that works in the local Ford parts department and he told me that they would either need the numbers off the transmission or you just have to pull the tail shaft and count the teeth. He aslo told me they used a 7,8,& 9 tooth gear on those years transmissions. So to double check I called the man I know at one of our top local transmission shops who rebuilt the C-4 in my 65 Comet and is really good and he told me exactly the same thing. Either take the numbers to Ford, or pull the tailshaft and count the teeth. I didn't even know they used a 9 tooth gear so now I have learnd something new from your post.

Ron