Speedo Gear Q's?
Moderator
Joined: Jan 2006
Posts: 1,779
Likes: 3
From: Waterloo IL
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.
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.
Laughing Gas
Joined: Dec 2007
Posts: 1,052
Likes: 2
From: Alvarado, TX
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?
Laughing Gas
Joined: Dec 2007
Posts: 1,052
Likes: 2
From: Alvarado, TX
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.
Laughing Gas
Joined: Dec 2007
Posts: 1,052
Likes: 2
From: Alvarado, TX
Try this. Sorry I can't remember what to enter for the speedo revolutions per mile though.
Speedo Gear Calculator - Wallace Racing
Speedo Gear Calculator - Wallace Racing
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Posting Guru
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From: Virginia Beach, VA
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Posting Guru
Joined: Jul 2011
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From: Virginia Beach, VA
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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Laughing Gas
Joined: Dec 2007
Posts: 1,052
Likes: 2
From: Alvarado, TX
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.
Laughing Gas
Joined: Dec 2007
Posts: 1,052
Likes: 2
From: Alvarado, TX
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.
Ford Parts Specialist
Joined: Nov 2006
Posts: 88,826
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From: Simi Valley, CA
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.
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.