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Discussion Starter #1
Well, I got a C4 and 3.40 gear in it NOW!!! HELLZ YEA!!!!:bigthumbsup:worship:heha:

Anyways, I need to know what spedo gear to put in since my gauge reads 10+ mph than what the REAL speed I'm going. I just read up that my driven gear is 7 tooth and I read in Vintagemustangs forum that some guy has 19-tooth with his 3.55. Curious if I should be using the same 19 tooth since a 2.79 uses a 16 tooth. I know I probably won't find a gear to an exact fitment of the 3.40 but need somewhere near it. What do you guys have??
 

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I changed mine but its original i think...C4 had a 16 tooth RH drive gear. The speedo seems to be right. I also got the orig rear differential. (again, i think) The code on door plate is blank but thinking its a 2:80

I was going to ask a trooper to check my speed once, but then i thought about it...what if he catches me at 55 in a 35 the next week? He will know my speedo is ok. So there's no way out of that ticket! :nono:
 

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Tire size comes into play also. In your case you mention 10 mph off but at what speed? If you could figure what percentage that is, just correct the number of teeth on the driven gear by that amount. That make any sense?
Dave
 

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Discussion Starter #4
Tire size comes into play also. In your case you mention 10 mph off but at what speed? If you could figure what percentage that is, just correct the number of teeth on the driven gear by that amount. That make any sense?
Dave

I saw a couple people posting different mathematical equations on this. I'm running 15" wheels on a 225/60/15. I know I was driving on the freeway at 72 which EVERYONE was passing me so I know I had to of been going around 55-60, and 45 on a regular road and people were passing me but not that fast. So assumption I was going 30 in a 35 mph zone. I believe I read last night that a C4 drive gear is a 7-tooth if I'm not mistaken which I heard that plays a role in changing spedo gear as well.
 

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Well, unless you change the drive gear, just do the math to find the driven gear teeth, don't worry about the drive gear.
Dave
 

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Discussion Starter #7
Cool. Thanks for lookin' out. I also heard it only takes like a couple minutes or so just to swap out. Wrench it out and replace with new one as I read.
 

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Discussion Starter #8
Thanks Hip for that link. That's easier than doing the math.:bigthumbsup Still I did the math. I just read on another forum was to divide the new gear with the old gear and times it by what the spedo gear that's in there already which is a 16 already. Then multiply the 16 by the answer after dividing both gears which is 19.something. So a 19 tooth is close as I can get .
 

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Easy. There is a clamp with one bolt attached to the tranny. Pull the end out after you remove the bolt. There is a small steel clip that holds the gear on the end of the cable assembly. DON'T lose this. Once the gear is on insert the assembly back into the tranny and re-install the bolt. Takes anout 10 minutes.
 

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It takes some patience too. There is something....E brake cable or exhaust, in the way, to get a good pull on it to get it out. Its easy tho. NPD has a mathematical equation in its book that tells you how to figure out what gear you need.
 

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Well, I got a C4 and 3.40 gear in it NOW!!! HELLZ YEA!!!!:bigthumbsup:worship:heha:

Anyways, I need to know what spedo gear to put in since my gauge reads 10+ mph than what the REAL speed I'm going. I just read up that my driven gear is 7 tooth and I read in Vintagemustangs forum that some guy has 19-tooth with his 3.55. Curious if I should be using the same 19 tooth since a 2.79 uses a 16 tooth. I know I probably won't find a gear to an exact fitment of the 3.40 but need somewhere near it. What do you guys have??
Hi,
I might be "that guy" running a 19 with a 3.55 rear. However, you do need to know your rolling tire dia. as well.
Check the TCI site for a calculator. There any number of calculators available, I just happen to use theirs.
 

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Hi,
I might be "that guy" running a 19 with a 3.55 rear. However, you do need to know your rolling tire dia. as well.
Check the TCI site for a calculator. There any number of calculators available, I just happen to use theirs.
If you are starting out with a fresh build and a lot of variables, charts are great. The OP has an existing combination and it's off by a percentage. You don't need to know anything other than that to figure out what the correct number of teeth are on the driven gear. If he is low by 10% then need 10% more teeth.
Dave
 

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Discussion Starter #13
Hey Ken

I think that is you on the Vintage Forum. I'm running 15" wheels on a 225/60. Isn't it a 16-tooth for stock tranny?
 

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Hey Ken

I think that is you on the Vintage Forum. I'm running 15" wheels on a 225/60. Isn't it a 16-tooth for stock tranny?
Hi,
It's quite possible if running 14" rims and a 3.00 rear. Those 225 X60X15s should be about 25.6".
 

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Discussion Starter #15
Thanks Ken. Not too experienced measuring tires like that, or how people measure tires on trucks (i.e. 33 inches). Thanks!
 

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I'm running 3.44 , BF Goodrich 255x15s and have a 23 teeth driven gear. I got a new C4 from TCI - based on needing a 23 teeth driven gear I'm assuming I have a 9 drive gear. I used my GPS to see how close the speedometer is to actual mine is pretty much right on now.

You can get your tire daimeter of MFG website usually. The calculator on TCI is pretty good - biggest variable causing some experimenting and the need to buy a couple different gears to try out is your drive gear. Try the calculation for 7,8,9 drive gears - see what you have now to try and eliminate possibilities and then buy the gears that match the calcs (they are pretty cheap and CJ Pony has free shipping)

In any case if you are running 10+ you definitely need a driven gear with more teeth.

TCI - TECHNICAL INFORMATION: Speedometer Gears
 

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Thanks Ken. Not too experienced measuring tires like that, or how people measure tires on trucks (i.e. 33 inches). Thanks!

Hey Bud,
Use this calculator. Plug your numbers into the appropriate fields on the left.
Tire size calculator

Keep this it forever come in handy.
Good Luck!
 

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Discussion Starter #18
Right on!!! I remember this from like 6 years ago when I bought my Titan. Not trying too hard, but I was trying to find this site. Thanks for the heads up Ken!
 

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Right on!!! I remember this from like 6 years ago when I bought my Titan. Not trying too hard, but I was trying to find this site. Thanks for the heads up Ken!
Hi,
Here's another one I've used and is actually better.
Calculators
Happy Motoring!
 
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