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Speedo Gear Q's?

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Old 03-24-2012, 02:54 AM
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Speedo Gear Q's?

My friend has a Bronco with 33 inch tires and 3.50 gears. It has a 17 tooth on it already.

Anyone know from experience what gear he would need?
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Old 03-24-2012, 08:57 AM
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I'd be real curious on this as well. I've got 33's and 3.50's on my 77 F-150.
So right now I am traveling farther than the odometer is reading. So if I get that corrected I'll get better gas mileage as well. lol.
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Old 03-24-2012, 10:41 AM
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I'm guessing that the speedo is reading slow so you need a gear with less teeth to speed it up. The lowest tooth count available for the driven gear is a 16 so you don't have a lot of options. What t-case does it have?
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Old 03-24-2012, 02:08 PM
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205 transfer case
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Old 03-24-2012, 03:06 PM
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I found a calculator on line a while back that figured this stuff out. I'll see if I can find it again when I get home. The drive gear in that case should have 7 teeth. If a 16 tooth driven gear doesn't do the trick you can install an 8 tooth drive gear from an NP208 that will give you more options.
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Old 03-24-2012, 09:56 PM
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Try this. Sorry I can't remember what to enter for the speedo revolutions per mile though.
Speedo Gear Calculator - Wallace Racing
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Old 03-26-2012, 04:00 PM
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Anyone know the speedometer revolutions per mile for our speedo's?
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Old 08-20-2012, 11:10 PM
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Bringing this back to the top. Need to figure out what he needs to get. He now has 35's on it with stock 3.50 gears.
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Old 08-21-2012, 04:57 PM
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Anyone know the speedo revolutions per mile then?
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Old 08-22-2012, 08:32 PM
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I believe it's 1000. I just put in my truck's specs using 1000 and it came out right.
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Old 08-22-2012, 10:31 PM
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I am also curious, my truck has 3.50s and 35s.
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Old 08-22-2012, 10:41 PM
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According to the calculator in the link above you need a 14 tooth driven gear, which doesn't exist. If you change to a 8 tooth drive gear from an NP208 it calls for a 16 tooth driven gear which is the lowest tooth count available.
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Old 08-23-2012, 12:36 AM
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Ya but how do you change it out to an 8? He just had the whole transfer case rebuilt and doesn't want to open it up.
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Old 08-23-2012, 01:46 AM
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It's about a 20 minute job. Remove the rear yoke then the cover that holds the rear seal. Then slide the output bearing off the shaft and you will see the plastic speedo drive gear in front of it. Swap the gear and put it back together. If he wants the speedo the be accurate it's either that or go to a speedo shop and see if they can make a converter box for it.
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Old 08-23-2012, 09:42 AM
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The nylon speedometer DRIVEN gears are the same since 1960, 16 -21 teeth are available.

But, if you have P-Metric radial tires, nothing you can do will make the speedo 100% accurate.

These tires are a dfferent shape than the radials used previously and most 1973/79 F100/350's came with bias ply tires as radial tires were optional.
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1979, 33, 35, 351m, chart, cylynder, gear, gears, inch, needed, rpm, speedometer, straight, tires, truck

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