Which driven gear do I need?
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Post Fiend
Joined: Oct 2009
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From: town of less than 200 ppl
Which driven gear do I need?
I've researched this before but never got around to correcting my speedometer. After using one of those nifty calculators I found online, I come up with needing an 18 tooth driven gear. Truck is a '74 F250, np435, 205 xfer case, 4.10 gearing, and has 37" tall tires. Does that sound about right to any of you? How close would a 16 tooth driven gear get me? Any help is appreciated.
Laughing Gas
Joined: Dec 2007
Posts: 1,052
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From: Alvarado, TX
The change in accuracy from gear to gear will vary from combo to combo however each tooth is generally worth about 2 to 3 miles an hour as read on the speedo. Therefore if you need an 18 tooth gear a 16 tooth would be off by 4 to 6 miles per hour. I have a 16 tooth gear in mine with 38 inch tires and 3.73 gears and it reads correctly. With your 4.10s it would be off significantly.
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Post Fiend
Joined: Oct 2009
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From: town of less than 200 ppl
Thanks for the replies guys. I don't know what is stock, as I have not had mine apart yet. I don't really want to take it apart until I have the replacement gear in hand.
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Post Fiend
Joined: Oct 2009
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From: town of less than 200 ppl
Please let me know what you find. I would really like to start daily driving my '74 to work, but I don't want to until I know I can get an accurate mileage count for checking fuel mileage, changing oil, etc.
Cargo Master
Joined: Nov 2010
Posts: 3,170
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From: darlington
18 tooth
hey brandon it will be awhile;till im on the road.Im putting motor in soon.Working on brake lines now.I would go with that 18,after what Fordworth said.
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Junior User
Joined: Dec 2013
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From: MT - The Last Best Place
call the ford dealer and ask for P/N c0dd-17271-b (18-tooth speedo gear) that's what I'm using with 4.10's NP205 and 315/75r16's (34.6" tall tires)
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Cargo Master
Joined: Nov 2010
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From: darlington
<TABLE border=0 cellSpacing=0 cellPadding=6><TBODY><TR><TD class=itemlink colSpan=2>Speedo driven gear 18 teeth (gold or yellow) </TD></TR><TR><TD vAlign=top width=60><IMG border=0 alt="New
1966-96 Ford Bronco
1973-96 Ford F100, F150, F250, F350
1984-90 Ford Bronco ll
18 teeth
gold or yellow
" src="http://broncograveyard.com/images/catalog/SPEEDOGEART.JPG" width=60 height=60> </TD><TD class=normal vAlign=top># 37192C
</TD></TR></TBODY></TABLE>Bronco Graveyard
1966-96 Ford Bronco
1973-96 Ford F100, F150, F250, F350
1984-90 Ford Bronco ll
18 teeth
gold or yellow
" src="http://broncograveyard.com/images/catalog/SPEEDOGEART.JPG" width=60 height=60> </TD><TD class=normal vAlign=top># 37192C
</TD></TR></TBODY></TABLE>Bronco Graveyard
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Post Fiend
Joined: Oct 2009
Posts: 8,674
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From: town of less than 200 ppl
Wow thanks guys. Wouldn't my speed-o still be off with an 18 tooth gear though? 406Angler mentioned that he was using an 18 tooth gear with 34 1/2" tires. I am running 37", so would I need a gear with more tooth count?
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