Speedometer correction
Posting Guru
Joined: Feb 2005
Posts: 2,499
Likes: 1
From: Missoula, MT.
pi D
Pi is 3.14 roughly times the Diameter, like my truck would be since i have 31 inch tires (well you should measure not go by the letters)
3.14 X 31 = 97.34 inches around.
Now if you wanted to find the area of a tire you would go 3pi times r squared x width.
which is 3.14 X radius squared X width so for me it owuld be 3.14 x's (31/2) squared in other words (3.14)X(15.5 X 15.5)X(10.5) which comes out to (3.14)X(240.25)X(10.5) which is 7921.0425 or 7,921.04 inches. I dont know why i posted the tire area, but thats how much air you need to inflate it, lol. plus you need so many PSI, so you can figure the rest out. lol
Pi is 3.14 roughly times the Diameter, like my truck would be since i have 31 inch tires (well you should measure not go by the letters)
3.14 X 31 = 97.34 inches around.
Now if you wanted to find the area of a tire you would go 3pi times r squared x width.
which is 3.14 X radius squared X width so for me it owuld be 3.14 x's (31/2) squared in other words (3.14)X(15.5 X 15.5)X(10.5) which comes out to (3.14)X(240.25)X(10.5) which is 7921.0425 or 7,921.04 inches. I dont know why i posted the tire area, but thats how much air you need to inflate it, lol. plus you need so many PSI, so you can figure the rest out. lol
Posting Guru
Joined: Feb 2005
Posts: 2,499
Likes: 1
From: Missoula, MT.
wait i forgot, you have to subtract the wheel size. so for me lets jsut say out of curiosity, not minus all the little nick nacks and stuff you take my wheel 15 X 8.5 so you multiply (3.14)(7.5)(7.5)(8.5) ok that gives us 2495.71125 so you take the orginal tire size calc and subtract 2495.71125 from it and it gives us (7921.0425)-(2495.71125) which equals 5425.33125 so therefore 5,425.33 cubic inches of air space. sorry i forgot that part earlier
Post Fiend
Joined: Mar 2004
Posts: 24,418
Likes: 3
From: San Francisco, Ca.
I came up with roughly 97.39" circumference.
Your original tires (P235x75R15) were 90.7"
I came up with a 7% difference.
Count the number of teeth on your speedo driven gear and multiply that by 107%.
That should give you the number of teeth you need.
Bring your old gear to the dealer and show them what you need.
Some Ford speedo driven gears are cut to spin in either clockwise or counter-clockwise direction.
Now, this is how I did my calculations on my truck.
Your original tires (P235x75R15) were 90.7"
I came up with a 7% difference.
Count the number of teeth on your speedo driven gear and multiply that by 107%.
That should give you the number of teeth you need.
Bring your old gear to the dealer and show them what you need.
Some Ford speedo driven gears are cut to spin in either clockwise or counter-clockwise direction.
Now, this is how I did my calculations on my truck.
Post Fiend
Joined: Mar 2004
Posts: 24,418
Likes: 3
From: San Francisco, Ca.
I thought you had 31s.
Sorry
18%
Multiply your number of teeth by 118%
I actually came up with 17% to 18% difference.
1% means nothing in this case.
How many teeth you got on the driven gear?
Sorry
18%
Multiply your number of teeth by 118%
I actually came up with 17% to 18% difference.
1% means nothing in this case.
How many teeth you got on the driven gear?
Last edited by sierraben; Oct 14, 2005 at 03:41 PM.
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Post Fiend
Joined: Mar 2004
Posts: 24,418
Likes: 3
From: San Francisco, Ca.
Now comes the fun part.
Either call of go down to the Ford dealer and see if they have a listing for a 23 or 24 tooth driven gear for your application.
If you can bring down the old gear and show them.
If for some reason they don't have a listing for a 23 or 24 tooth gear, you may have to do a search for a "speedometer gear reducer".
Also, maybe call your local tire shop and see what they say.
Either call of go down to the Ford dealer and see if they have a listing for a 23 or 24 tooth driven gear for your application.
If you can bring down the old gear and show them.
If for some reason they don't have a listing for a 23 or 24 tooth gear, you may have to do a search for a "speedometer gear reducer".
Also, maybe call your local tire shop and see what they say.
Post Fiend
Joined: Mar 2004
Posts: 24,418
Likes: 3
From: San Francisco, Ca.
Good luck.
Don't forget, some driven gears are designed to spin in one direction, while others are designed to spin in the other direction.
I ran into that problem, so I had show them my original gear.
Hope you get a parts guy who knows what he's doing and takes the time to help you out.
Don't forget, some driven gears are designed to spin in one direction, while others are designed to spin in the other direction.
I ran into that problem, so I had show them my original gear.
Hope you get a parts guy who knows what he's doing and takes the time to help you out.
Fleet Owner
Joined: Sep 2005
Posts: 28,609
Likes: 18
From: Lake Tahoe, Nevada
Originally Posted by jpjahn
The new ones are 110 inches in circumference. The old tires that were on there were not stock though. They were 31X10.5X15, so I should use the stock circumference for the 29 inch tire. If my calculations are correct, a 29 inch tire would be 91.1 inches around. Does that sound right? So the difference is 18.9 inches. What do i do with that number now?
Post Fiend
Joined: Mar 2004
Posts: 24,418
Likes: 3
From: San Francisco, Ca.
Bdox, I think I gave him the wrong info. It's been a long day.
jpjahn, Sorry about that.
I guess my speedo situation was a different because of the rear end ratio change.
So I guess you need a either a 16 or 17 tooth driven gear. (20 x 17%=16.6)
jpjahn, again sorry about that.
EDIT:Bdox, Are you absolutely sure?
The more I think about it, the more I want say a bigger gear.
Here's how I see it.
With the bigger tires, say you're actual speed is 60MPH. Isn't the speedometer reading a much higher number? The rear wheels are spinning the speedo faster.
So wouldn't the drive gear have to be bigger to slow down the speedo?
Now I'm confused.
Aw shoot, I think your're right.
jpjahn, Sorry about that.
I guess my speedo situation was a different because of the rear end ratio change.
So I guess you need a either a 16 or 17 tooth driven gear. (20 x 17%=16.6)
jpjahn, again sorry about that.
EDIT:Bdox, Are you absolutely sure?
The more I think about it, the more I want say a bigger gear.
Here's how I see it.
With the bigger tires, say you're actual speed is 60MPH. Isn't the speedometer reading a much higher number? The rear wheels are spinning the speedo faster.
So wouldn't the drive gear have to be bigger to slow down the speedo?
Now I'm confused.
Aw shoot, I think your're right.
Last edited by sierraben; Oct 15, 2005 at 02:33 AM.
Fleet Owner
Joined: Sep 2005
Posts: 28,609
Likes: 18
From: Lake Tahoe, Nevada
When you install larger tires, your speedo reads too slow. So you now need to speed up the speedometer to match your speed. Since the speedo drive gear is the "driven" gear it needs to have fewer teeth to drive the speedo faster.