Speedometer Gear
Caps Lock Was On!
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Joined: Jan 2001
Posts: 56,971
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From: Virginia
Originally Posted by ri_truck_guy
The simple formula doesn't work because there was also a change in the tire size...the original gearing was based on a stock 235/75/15 tire, which was about 29 inches in diameter. The new tire size is 36 inches. The tire size is another variable.
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Junior User
Joined: Jan 2007
Posts: 60
Likes: 0
I did a little more research and found ri_truck to be right with his gear choice. the formula I found was @ this web site http://www.sccoa.com/faq/speedgr.html.
this is the formula the site uses. Speedometer Gear Teeth = (Drive Teeth * Rear End Ratio * 20.2) / Tire. Thanks for all who posted with all of the helpfull info, and thanks to BIG JOHN for todays grammar lesson!
this is the formula the site uses. Speedometer Gear Teeth = (Drive Teeth * Rear End Ratio * 20.2) / Tire. Thanks for all who posted with all of the helpfull info, and thanks to BIG JOHN for todays grammar lesson!
Posting Guru
Joined: Jun 2002
Posts: 2,091
Likes: 3
From: Suffolk, VA USA
Dave,
From what I could gather there were two tire changes on the truck. In 89, it should have rolled out of the factory on 235/75/15s. Then someone put 30s on it, then he put 36's on it. My understanding from the various posts was that he ran the truck with the 3.08s with both the 30s and 36s, and the speedometer was off to different degrees with both setups. Calculating from the stock setup was only important to figure out the constant.
It went something like:
Stock Tire x Stock Speedometer / New Tire / Old Gears x New Gears = New Speedometer...in this case
28.9 x 18= 520.2/36 = 14.45/3.08= 4.6915 x 4.56 = 21.39
or, the red gear...
Bill
86 F150 4x4 5.0
From what I could gather there were two tire changes on the truck. In 89, it should have rolled out of the factory on 235/75/15s. Then someone put 30s on it, then he put 36's on it. My understanding from the various posts was that he ran the truck with the 3.08s with both the 30s and 36s, and the speedometer was off to different degrees with both setups. Calculating from the stock setup was only important to figure out the constant.
It went something like:
Stock Tire x Stock Speedometer / New Tire / Old Gears x New Gears = New Speedometer...in this case
28.9 x 18= 520.2/36 = 14.45/3.08= 4.6915 x 4.56 = 21.39
or, the red gear...
Bill
86 F150 4x4 5.0
