Spedo Gear Size?

NMBronco77 · Aug 25, 2004

Howdy,

How do I know what size (# of teeth) speedometer gear to use, when I change from stock tires to 33s, in order to have the correct MPH reading? The gear ratio of my rear diff is 3.55/1.

Thanks,

Pat Ruminer in NM
77 Ranger

Seabronc · Aug 26, 2004

The easies way to caqlibrate your speedometer is to get someone to go out with you on the highway. Hold an indicated speed of 60 for one mile and time it with a stop watch.

Then use this formula to find your actual speed Actual MPH = 3600/Time then find the % error with the formula % error = MPH error/Actual MPH.

ex. Example:

you clock your speed from mile marker to mile marker as 56 sec

3600/56 = 64.29 MPH actual, an error of 4.29 MPH

Percent Error = MPH error/actual MPH (4.29/64.29 = .066 or 7%)

Count the number of teeth on your existing speedo gear. One tooth will change your indicated speed approximately 7% on a C6, not sure what is in your truck.

Each tooth removed raises the speedometer indication, each tooth added lowers the speedometer indication.

Good luck,

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Broncoholics · Aug 31, 2004

Call a few spedometer shops and they might be able to tell you by the gears and tire combo you have.
Plus you could buy a few different gears since they are cheap and easy to change and do the trial and error method.

Toby the Dancing Bronco · Oct 1, 2004

You are good at making me think of other questions no one but you can answer! I just rebuilt my Dana 20, and found my speedo-driven gear had joined an unholy alliance with a bearing, and well, it wasn't very pretty. All is forgiven, now, after a few bench hours and hitting paydirt on an output shaft and rear housing. My question is this: the speedo drive gear was an ivory color, as I remember it. When I put 31's on it, the speedo was off a bit. Now I want to go to 33's. What number of teeth, or color of driven gear should I look for? I had been driving without a speedo for a couple of years, but always knew how fast I was going because all of the rattles and vibration would sort of come into a harmonic convergence and I knew that was fast enough. After the D20 rebuild, theres not that much shaking going on... I kind of miss it, but now I can hear myself think, and I am thinking "How fast am I going?"
Thanks!

Seabronc · Oct 1, 2004

I took a look at the ones I have, and the ivory one is 17 teeth, yellow is 18 teeth, and the maroon one is 16 teeth, (color is not 100% reliable, but the tooth count is on the gear in fairly small print, either on the front end or on the side behind the teeth). They are cheep and you should be able get the Ford dealer counter guy to check his stock. Get one and do what I said in an earlier post and adjust from there. I would guess 1 to 2 teeth down.

Goood luck,

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4xfan78 · Oct 5, 2004

I changed mine a while back, all I did was went to the ford dealership and at the parts counter and told them what I needed. They took the gear ratio tranny and tire size and in about 5 min he came back with the correct gear. Not saying anybody elses ideas are not good but sometimes the simplest ways work alot easier.

Seabronc · Oct 6, 2004

4xfan78 is right. If you get the right counter guy [-o< , he can be a great deal of help and save you some time and money. However, I have only found 1 in a 30 mile radius

that goes the extra mile and has found me some, "no longer stocked", parts. He will do almost anything for a fellow Bronco lover. On the other hand, you may have to do the thinking and calculations for your self :wacko: .

Good luck,

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