My speedometer is off --- a lot. At 50 mph my speedo reads 60-65. The previous owner must not have addressed that when he did other things to the car. The car has a TCI Streetfighter C4 (I doubt that's why the speedo is off), 16-inch tires (more likely the issue) and a 3.70 (or 3.79 - I have it written down at home) gear in the rear end (most likely the issue).

The first time I drove the car to work (first drive of more than 10 miles) the speedo cable started squeeking. On the way home, it broke. I pretty sure its the cable because there was 3-in-1 oil on my shoe when I got home. Since I'll be replacing the cable this weekend, I might as well get the right gear for the end that plugs into the transmission.

Does anyone know how to decide which gear I need?

 

let us know the tryesize (ex 225/60R16). with that and the exact real axle ratio we should see how far away you are from stock, which might give a more exact idea of what ratio the gear needs to be

 

3.70 gear in differential.

Tires are 225/55R16, which computes to 25.74" tire height.

 

There is a formula for calculating the right driven gear based on tire rotations per mile, rearend gear, and speedo drive gear in the tranny. NPD has the formula posted in their catalog on the page with the speedo grears (page 139 of the 2009 catalog).

Edit: Formula is:

# of driven gear teeth = (# of drive teeth * Axle ratio * Tire rev per mile)/(1000)

 

With a 6 tooth drive gear the math is:

(6*3.70*783)/1000 = 17.38

With a 7 tooth drive gear the math is:

(7*3.70*783)/1000 = 20.28

Unfortunately, the drive gear tooth count cannot be determined unless you pull the tranny apart or ask the builder what they used. FWIIW, I have a 7 tooth in my toploader David Kee built me.

 

Duplicate post.

 

You should be able to see the drive gear by looking in the hole in the transmission where the speedo cable goes. I'm not super familiar with C4's, but manuals have color coded drive gears rather than having them cut directly into the tailshaft. If you can get the color, call Glazier's with your info and they'll set you up with the right driven gear.

 

The autos have also color coordinated gears. I swapped my C4 for an aod and falied to swap the gear and got a ticket for speeding when I thought I was right on the speed limit (81 in a 75).

I went to the ford dealer, and they don't have those gears anymore. So a local trans place did. Here is what I would do:

Pull the gear out and look at the color. Search online for the color to find the number of teeth (or count but that is challenging). Your correction (I think) is 65 / 50 = 1.3 Now if you have a 7 tooth gear, multiply that by 1.3 and you come up with about a 9 tooth gear.

I am either 100% correct or 100% wrong. I think I went from a 12 tooth to an 11 tooth to correct my underestimation problem.

 

I am definitely wrong about the number of teeth. Go to this page and about half way down and you will see picture of the ford automatic speedo gears. Looking at it now, I think I went from a 19 (tan) to an 18 (grey).

http://www.transmissioncenter.net/sp...n_______va.htm

So if you have a 16, multiply that by 1.3 and you will get 21. This is just a semi educated guess.

 

See page 139 NPD Catalog & read the RED text..

Isn't it safe to say that the 6-tooth drive gear was used in 3.50 to 4.30 gears and the 7-tooth from 2.70 to 3.50 gears? If the car has 3.79 gears they were changed and likely the drive gear wasn't. Correct?

I just ordered a 7-tooth drive gear and an 18-tooth LH driven gear for our toploader. 3.25 gears and 245/50ZR16tires. I think thats correct.