Speedo Calibration
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5th Wheeling
Joined: Aug 2021
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Speedo Calibration
Hello, just wondering if I could get some help with my situation. My truck has 35x12.5R20’s and I knew my Speedo was off so I decided to change it and without thinking I didn’t realize the rim size would matter so I found a forum and used the numbers for a 35x12.5R15 and when I put in that calibration my speedo is now reading 10 mph high. I read some forums that on how to do the math but I can not figure it out and was thinking someone on here could figure the numbers!
FTE Emeritus
Joined: Jan 2006
Posts: 2,288
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From: springfield Missouri area
Hello, just wondering if I could get some help with my situation. My truck has 35x12.5R20’s and I knew my Speedo was off so I decided to change it and without thinking I didn’t realize the rim size would matter so I found a forum and used the numbers for a 35x12.5R15 and when I put in that calibration my speedo is now reading 10 mph high. I read some forums that on how to do the math but I can not figure it out and was thinking someone on here could figure the numbers!
I found that the process to change the speedometer is correct but the math to figure a new CC is not accurate. This is how I got my speedometer to read accurate.
Before doing any adjustments use gps unit and clock speed at 75, record what speedometer reads. In my case it was about 70. See what CC currently reads, again in my case it was 742 or thereabouts. It's been awhile so can't remember exact number.
In your case adjust the CC to 842 then record what speedometer reads at 75 again. Using the Forum CC numbers moved my speedometer about 5 mph the other direction, so my speedometer now read 80.
Do your own math and move CC I bet your speedometer will be perfect. This worked for me. Good luck, I did it so it's definitely not rocket science.
I didn't do well on that write up. Go 100 points above what the CC currently is, that will put you close to your calculations. Record what the speedometer reads before and after the adjustment at a consistent 75 mph using gps. You'll then be able to figure how many CC units to move 1 mph. It only takes twice to get it dialed in.
The initial move could be any number you just need to know the CC number before you move it. The 100 just makes the math easier.
Cargo Master
Joined: Nov 2009
Posts: 2,190
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Hello, just wondering if I could get some help with my situation. My truck has 35x12.5R20’s and I knew my Speedo was off so I decided to change it and without thinking I didn’t realize the rim size would matter so I found a forum and used the numbers for a 35x12.5R15 and when I put in that calibration my speedo is now reading 10 mph high. I read some forums that on how to do the math but I can not figure it out and was thinking someone on here could figure the numbers!
Fleet Owner
Joined: Oct 2005
Posts: 20,789
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From: Kentucky
What calibration constant did you put in the PSOM?
As noted earlier rim size does not affect the equation for the PSOM calibration constant. Tire revolutions per mile and the number of teeth on the tone ring in the rear axle are the main variables to account for. A 35" tire used on a F-150/Bronco w/8.8" rear axle (108 teeth) will have a different constant on a F250/350 or a swapped F150/Bronco using a Sterling 10.25/10.5" rear axle (120 teeth).
As noted earlier rim size does not affect the equation for the PSOM calibration constant. Tire revolutions per mile and the number of teeth on the tone ring in the rear axle are the main variables to account for. A 35" tire used on a F-150/Bronco w/8.8" rear axle (108 teeth) will have a different constant on a F250/350 or a swapped F150/Bronco using a Sterling 10.25/10.5" rear axle (120 teeth).
Tuned
Joined: Apr 2003
Posts: 394
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From: 13th Colony
Mental note, tires are not the advertised height. You might should measure the actual height to verify. They are usually shorter than what is stamped on the side when installed. No rims shouldn't matter if using US tire sizes. If using metric, then the rim size is part of the calculation for tire height (diameter).
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bradphx
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