Speedo Adjustment for 35s
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Speedo Adjustment for 35s
My 77 F-150 4x4 (351M w/ 4 speed & 4.11 gears) will be getting 35s. How do I adjust the speedometer to take the new tires into account? I've heard there is a gear in the speedo or tranny you can change but I don't know any details.
Is this hard to do? How/where is it done?
Is this hard to do? How/where is it done?
Last edited by Ken2600; Feb 26, 2003 at 09:35 AM.
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Speedo Adjustment for 35s
Wow, that's a ton of info on that page. I've bookmarked it for the future.
Even though it had the speedo error calculations based on tire size, I'm still not sure what I can change on my truck so that the speedometer reads correctly.
Anyone else?
Even though it had the speedo error calculations based on tire size, I'm still not sure what I can change on my truck so that the speedometer reads correctly.
Anyone else?
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Speedo Adjustment for 35s
crawl underneath your rig. look for the speedo cable. where it installs into the back side of the T-case it is held in by a 7/16 hex head bolt. undo the clamp and pull the cable out. there will be a plastic driven gear. this is what you change to recalibrate the speedo.
Eric
Eric
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Speedo Adjustment for 35s
Thanks Alpo! Now I just need to figure out how many teeth my new gear will need and where to find it.
Does anyone know the number of teeth off the top of their head for 4.11 gears and 35" tires or at least the math to figure it out based on how many teeth it has now?
Also, is that gear a piece you can get from an aftermarket company or from Ford?
Does anyone know the number of teeth off the top of their head for 4.11 gears and 35" tires or at least the math to figure it out based on how many teeth it has now?
Also, is that gear a piece you can get from an aftermarket company or from Ford?
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Speedo Adjustment for 35s
you need to know what you have before recalculating the new tooth count.
Either Ford or in the junk yard for the speedo gears.
I just collect them over the years from all the cars/trucks I've owned.
Eric
p.s. What size tires did it have before jumping to 35" . also measure the old (if possible) and the new ones. The actual size is what matters. Your looking for the % difference between the old and new. More than likely you will drop one tooth in tooth count (i.e. 17 to 16 tooth). You need to speed up the cable speed to compinsate for the drop in rpms with the larger tire.
Either Ford or in the junk yard for the speedo gears.
I just collect them over the years from all the cars/trucks I've owned.
Eric
p.s. What size tires did it have before jumping to 35" . also measure the old (if possible) and the new ones. The actual size is what matters. Your looking for the % difference between the old and new. More than likely you will drop one tooth in tooth count (i.e. 17 to 16 tooth). You need to speed up the cable speed to compinsate for the drop in rpms with the larger tire.
Last edited by alpo23; Mar 2, 2003 at 06:48 PM.
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Speedo Adjustment for 35s
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Speedo Adjustment for 35s
Dennis, you've lost me there (again). My brain doesn't work as well as yours I guess. 
As for old vs. new tire size. It's originally set up for 235/75/15s which (I think) are about 29 inches high. I haven't gotten the new tires yet but I expect them to be about 35 inches high.
As for old vs. new tire size. It's originally set up for 235/75/15s which (I think) are about 29 inches high. I haven't gotten the new tires yet but I expect them to be about 35 inches high.
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Speedo Adjustment for 35s
First we need to know what color the speedo gear is in there.
I can get you close.
There are many calibration sites on the web.
We might even have one here in the tech articles.
I'm surprised Seans page didn't have the calculator.
http://www.boss302.com/speedo.htm
I can get you close.
There are many calibration sites on the web.
We might even have one here in the tech articles.
I'm surprised Seans page didn't have the calculator.
http://www.boss302.com/speedo.htm
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Speedo Adjustment for 35s
I finally got under the truck today.
My driven gear is black. On the end it says 20R (it also has an 8 on the opposite side but I'm not sure what that is). So I guess the 20R means 20 teeth, right hand twist maybe?
This is the original gear and it came from the factory with 235/75R/15s which I think are about 29 inches high.
What driven gear will I need for 33" tires and what would I need for 35" tires? I might have to go with 33s instead of 35s depending on the height of my garage doors.
My driven gear is black. On the end it says 20R (it also has an 8 on the opposite side but I'm not sure what that is). So I guess the 20R means 20 teeth, right hand twist maybe?
This is the original gear and it came from the factory with 235/75R/15s which I think are about 29 inches high.
What driven gear will I need for 33" tires and what would I need for 35" tires? I might have to go with 33s instead of 35s depending on the height of my garage doors.
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Speedo Adjustment for 35s
you need to take a tape measure and measure as close as you can the distance from the centerline of the axle straight down to the ground. This is rolling radius. Then do the same for the new tires. Divide the new radius by the old (eg. 16"/14"). for this example it will equal 1.143. This means there is a 14% difference between the new and old tire size. You have a 20 tooth gear. Multiply 20 x .14 = 2.8 rounding to the closest whole integer it is 3. This means to get the closest you need a 17 tooth gear. THIS IS ONLY AN EXAMPLE.
So go measure what you have now. When you get the new tires you can finish the calculations.
Good luck.
Eric
So go measure what you have now. When you get the new tires you can finish the calculations.
Good luck.
Eric
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Speedo Adjustment for 35s
Thanks again. I'll have to measure a 235 to figure out what the gear that is in it was setup for. It's got 30x9.50x15s on it now and I think they're about an inch higher than the stock 235s.
Then once I get my new tires, I'll figure it out and try to find my new gear.
Then once I get my new tires, I'll figure it out and try to find my new gear.
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