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Speedometer gearing

jbtherat

New Member
Joined
May 27, 2014
Messages
38
Loc.
Houston
Alright, maybe a dumb question. Since I'm using taller tires, does the speedometer gear in both the transmission and transfer case need to be changed out? I haven't started the rebuild on the tranny yet.
4R70W and NP205. Plans are for 35" tires and 4.88 or 5.13 gears.
Drive and driven gear??? WTF!
I'm reassembling my 205 and need to be schooled before I put it back together.
Thx for the help.
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,833
Well, there is no speedometer gear in the transmission. But if you find one, you better change it right away.

Before you put the 205 back together, count the number of teeth on the driving gear. The one that mates to the easily changed driven gear on the cable. It will help you figure out the right tooth count for the driven gear.
 

suckerpunched

Sr. Member
Joined
Aug 24, 2006
Messages
882
no idea how many teeth are on the 205 drive gear but if it's 6 or 7, heres a chart that will help
 

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  • speedo gear chart 2.JPG
    speedo gear chart 2.JPG
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DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,345
And since you're figuring speedo stuff, don't just go by the tire size on the sidewall. Measure the exact diameter of the tire on the ground with the full weight of the rig on top of it.
Since this is not easy with a rig you're still building, maybe someone here already has that info for a particular tire.

So let us know what tire exactly you'll be running. I've had 35" tires that were from 33.5" to 34.25" tall when sitting on the ground. Also called rolling radius, that's the distance from the center hub to the ground with the full weight of the vehicle on it.

Paul
 

RPM289

Sr. Member
Joined
Jan 4, 2008
Messages
835
Well the chart doesn't help me any LOL I have 38.5 tall tires and 4:11 assuming stock gear in T case.
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,833
Well the chart doesn't help me any LOL I have 38.5 tall tires and 4:11 assuming stock gear in T case.

It does help you. It lets you know that there is no easy gear that will work. You now know that you have to fix it a different way.
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,345
Unless the tires are actually 38.5" tall, the 16t gear will get you pretty close with the 7 tooth drive gear. Which hopefully the 205 has.
Kind of hard to count the numbers on those spiral spline type gears, but it can be done.

And since 16t is as small as they make for the Ford setups, that's your choice.
Because 4.11 is a pretty big mismatch for your size tire, is there a gear swap in your future? Or are you happy with the performance of the setup so far? Seems like it would be kind of a dog off the line with that setup, but depends on other factors as well. Such as what transmission, how you drive, the weight of the rig, etc.
But that size tire is fairly screaming for a lower gear ratio.

Good luck either way. Might need a temporary reduction box inline with the cable, or whatever they use these days.
Or a new Dakota Digital cluster with electronic adjustable speedo! That would work sweet. Hey, it's only money either way, right?;D

Paul
 

RPM289

Sr. Member
Joined
Jan 4, 2008
Messages
835
Unless the tires are actually 38.5" tall, the 16t gear will get you pretty close with the 7 tooth drive gear. Which hopefully the 205 has.
Kind of hard to count the numbers on those spiral spline type gears, but it can be done.

And since 16t is as small as they make for the Ford setups, that's your choice.
Because 4.11 is a pretty big mismatch for your size tire, is there a gear swap in your future? Or are you happy with the performance of the setup so far? Seems like it would be kind of a dog off the line with that setup, but depends on other factors as well. Such as what transmission, how you drive, the weight of the rig, etc.
But that size tire is fairly screaming for a lower gear ratio.

Good luck either way. Might need a temporary reduction box inline with the cable, or whatever they use these days.
Or a new Dakota Digital cluster with electronic adjustable speedo! That would work sweet. Hey, it's only money either way, right?;D

Paul

I was told when I bought the bronco it had 4:56 but marking drive shaft and tire looks like 4:11 without pulling it out. I want to drop back to a 35 or 36 tire but may change gears later. I have been eyeballing the Procomp speedo :) :) It is a little grumpy pulling off but glass tub helps on the weight side.
 
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