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Discussion Starter #1
I'm currently running 35" mt/rs (soon to be 37s) and I recently regeared my truck to 4.88s after my sas. As of now my speedo is wayyyyyy off. Anybody offer advice as to the best solution for this?
 

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You're running 35's on a Miata and you're worried about your correct relative speed?:flipoff2:


Can you reprogram the onboard computer? Or install a correct speedo gear?
 

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change your speedometer gear. i have a formula that will help determine which tooth count you need. i will try to find it and post it up
 

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if its a new truck, it uses a vss in the rear end, and has to be recalibrated
 

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Discussion Starter #6
shit, sorry, its an 88 bronco.
i think you are right tranny boy, please let me know asap-thanks!
 

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if you figure the exact gear you need, napa, ford, they can get them.
 

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found it. if the speedometer was accurate before gear change use the following
# teeth X new gear ratio divided by old ratio will equal new speedo gear count
e.g. 18 X 4.10 divided 3.55=20.79(21)
here is one for one if it was not accurate before
gear ratio X speedometer drive gear divided by tire size X 20.2(constant)= #of teeth on driven gear
e.g.
3.73(ratio) X 7(teeth) div. 32(tire size) X 20.2 = 16.48 (16 or 17 teeth)
you should be able to find these different gears at any parts store , dealership or your local transmission shop. hope this all helps
 

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Discussion Starter #11
That formula says I need 31 teeth on the gear, can that be correct?
88 Bronco
4.88 gears
35" tires
I believe 11 teeth on the current gear
 

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and it says I need 34??!

don't think it it quite right, eh?
 

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What I did when I went from 31s to 35s was find one of those speedo calculators you can find just about anywhere on the web. I calculated the more common speeds (25, 35, 70 etc) and wrote them on a stickynote and taped it to my dash :D

First comment anyone makes when they get in my truck for the first time: "hey what's those numbers for??" :laughing:

It works, I have yet to get a speeding ticket.
 

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Heres the gear formula
Old (stock) tire diameter X old speedo gear teeth
new tire diameter

= new speedo gear teeth

16 tooth Wine
17 white
18 yellow
19 natural
20 black
21 red
 

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Step 1: Find out how far off your speedo is.
Drive on the highway for 10 miles (according to mile markers) and note how far your odometer advances. Divide the odo reading by 10 and that will tell you what percentage your speedo is off.

Example: If you drive 10 miles and your odometer advances 8.75 miles, 8.75 / 10 = .875. So your speedometer is reading 87.5% slow.

Step 2: Find out what speedo gear you need.
Remove the speedometer driven gear from the transfer case. Do this by following the speedometer cable down to the transfer case. There will be 1 bolt to remove, and the gear should come out with the cable. Count the number of teeth on the gear. Multiply the number of teeth by how far off your speedo is and this will give you the gear you need.

Example: If you find that you have an 18 tooth gear and your speedometer is 87.5% off, 18 * .875 = 15.75. So you need a 16 tooth gear which is the closest to the result.

Step 3: Locate the correct gear and install it.
Try your local dealer, and if they don't have them, you may need to go to a company that specializes in obsolete parts. Pull the metal clip off the gear and the gear should come right off the cable. Install the new one and insert it back into the transfer case and you're done.

Step 4: Test it!
I tried this fix, the example numbers are what I came up with. When I drove the same 10 miles on the highway my odometer advanced 9.9 miles! I have also tested it by driving next to cars with digital speedometers and compared to my GPS unit, and it is almost dead on!

Please note that most highway markers are not perfect, but over 10 miles it should be pretty close. You can use a longer distance for more accuracy.


Speedometer Driven Gear Part Numbers Part Number # of teeth Gear Color
C0DZ-17271-A 16 teeth wine
C3DZ-17271-C 17 teeth white
C0DD-17271-B 18 teeth gold or yellow
C0DZ-17271-B 19 teeth natural color
C1DZ-17271-A 20 teeth black
C4OZ-17271-A 21 teeth red

I got my gear from Green Sales Company. http://greensalescompany.com/
 

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one thing i forgot to mention is that the tire size needs to be the true tire measurement not the manufactures.
 
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