SPEEDO GEARS!!!
Postmaster
Joined: Mar 2008
Posts: 2,703
Likes: 7
From: Des Moines, Iowa
Ok so how do you calculate tire revolutions per mile???
My 88 F-250 (thumper) has 31in tires (265/75R16), 3.55 rear gear, and the origional speedometer gear was a 19 tooth pink gear... origional tire size according to the door jam is 215/85R16... so what gear do i need now with the bigger tires?
From what i've read i should knock it down to the yellow 18 tooth gear then...??? how many teeth up or down can you move before needing to change the driven gear tooth count? Just asking because both my trucks are in need of this fix to be more accurate on the odometer and for mpg...
My 88 F-250 (thumper) has 31in tires (265/75R16), 3.55 rear gear, and the origional speedometer gear was a 19 tooth pink gear... origional tire size according to the door jam is 215/85R16... so what gear do i need now with the bigger tires?
From what i've read i should knock it down to the yellow 18 tooth gear then...??? how many teeth up or down can you move before needing to change the driven gear tooth count? Just asking because both my trucks are in need of this fix to be more accurate on the odometer and for mpg...
Fleet Owner
Joined: Aug 2005
Posts: 23,668
Likes: 301
From: Easton,Ks
Most of the time you can go to your tire Mfg. web site and it will tell you the tire revolutions per mile.
Again you would need to get the tire revolutions per mile before you would know.
I think all the auto transmissions have the same driven gear tooth count. Likewise I think all the manual transmissions have the same driven gear tooth count.
So you should be able to use any of the gears listed in the chart above.
So you should be able to use any of the gears listed in the chart above.
Lead Driver
Joined: Nov 2002
Posts: 9,557
Likes: 44
From: Concord, NC
I think all the auto transmissions have the same driven gear tooth count. Likewise I think all the manual transmissions have the same driven gear tooth count
I swapped to a ZF in my '87 and had to go from the factory 18 tooth driven gear to a 21 tooth because the drive gear was 7 factory, and 8 on the ZF I have.
Fleet Owner
Joined: Aug 2005
Posts: 23,668
Likes: 301
From: Easton,Ks
Yes I was wrong above the automatic would be a 7 or 8 tooth drive gear.
Manual transmission or transfer case would be a 6, 7 or 8 tooth gear.
For more information go to this URL:
http://www.transmissioncenter.net/sp...n_______va.htm
/
Manual transmission or transfer case would be a 6, 7 or 8 tooth gear.
For more information go to this URL:
http://www.transmissioncenter.net/sp...n_______va.htm
/
New User
Joined: Apr 2010
Posts: 17
Likes: 0
From: Western Colorado
OOPS, Forgot to mention my truck has 4.10's and 235/85-16's. It sounds as though I'll have to pull the speedo cable and look at the drive gear color. I was hoping Ford used 1 drive gear in all it's trucks. That's what I get for... Kelly.
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