Speedo Gear

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pyro
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Speedo Gear

Post by pyro » Fri Oct 03, 2014 6:26 am

Where can I find the correct speedo gear for 3.31's and 33" tires? I have the np208
'81 J10 Laredo 258/T176
'84 CJ-8 258/T176

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carnuck
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Re: Speedo Gear

Post by carnuck » Fri Oct 03, 2014 6:47 am

The gear tooth number will depend on what your original was. NP208, 219, 228, 229 use the same style gear and probably Dodge NP208s too. I use http://4lo.com/calc/geartire.php and input the tire sizes (original tire size: example 27", new 33" and 3.31 ratio) If the tcase and axles are original ratio, then you want the gear from a rig with 2.72 (whatever it says for the effective ratio) That's what I currently have for the Honcho as well.
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derf
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Re: Speedo Gear

Post by derf » Fri Oct 03, 2014 8:33 am

I started to fiddle with a calculator that would spit out the numbers you need but it's not ready yet.

The equation is: 20.168 x axle gear ratio x teeth on the drive gear / tire diameter = speedo gear you want.

The NP208 in the Jeep has 13 teeth on the drive gear.

So you want 20.168 x 3.31 x 13 / 33 = 26, rounded down. So you want a 26 tooth gear. If your 33 runs small, you may want a 27 tooth.


Another simple way to get the right gear is to take out the one that came with your vehicle stock and count the teeth. Then just do simple math:

old gear teeth x new tire size / old tire size = new gear teeth.

Also:

old gear teeth x old axle ratio / new axle ratio = new gear teeth.

If you change both axle ratio and tire size, you do the tire calculation then plug that number into the axle calculation. The end result is the number of teeth you want.
KE0ITX

Check out my gear ratio calculator

If you can't fix it with a hammer or duct tape you have an electrical problem.

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derf
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Re: Speedo Gear

Post by derf » Fri Oct 03, 2014 8:50 am

And because I'm bored, I made a chart for easy reference in case anyone finds this thread and wants to know a different combination:

Image
KE0ITX

Check out my gear ratio calculator

If you can't fix it with a hammer or duct tape you have an electrical problem.

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pyro
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Re: Speedo Gear

Post by pyro » Wed Oct 08, 2014 9:39 am

Thanks that helps out a ton! Now I just need to find the damned gear somewhere.
'81 J10 Laredo 258/T176
'84 CJ-8 258/T176

Florida FSJ's Click Here!!!


chevelleguy
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Re: Speedo Gear

Post by chevelleguy » Wed Oct 08, 2014 1:50 pm

They were also the same style as used in the Chrysler automatics. Try your local independent transmission shop for a used one if you know your tooth count, make sure it doesn't have a grove worn in the seal area or it will leak. You can probably get a new one online too.

Here you go.
http://www.ebay.com/itm/MOPAR-SPEEDOMET ... 21&vxp=mtr
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