Help me figure this out!

I went from the standard 33's on my 2018 3500 about 5,000 miles ago to 35's. I bought a Rough Country tire calibrator to make sure the speedometer was accurate. After dialing it in as closely as I could, the speedometer was only 1 to 2 mph faster than reality at highway speeds. Everything was fine for about 2,000 miles and then it threw a tire sensor code. I updated the calibrator (I know--should have done that first), cleared the DTC and everything was fine for a while and, of course, it threw the same code. Here I am at about 5,000 miles and I've thrown the same tire sensor code 5 times. Out of frustration, I decided to set it back to stock and just deal with the speedometer error. Well...now the speedometer is dead accurate! Um...how? Thoughts?

alejes02 said:

I went from the standard 33's on my 2018 3500 about 5,000 miles ago to 35's. I bought a Rough Country tire calibrator to make sure the speedometer was accurate. After dialing it in as closely as I could, the speedometer was only 1 to 2 mph faster than reality at highway speeds. Everything was fine for about 2,000 miles and then it threw a tire sensor code. I updated the calibrator (I know--should have done that first), cleared the DTC and everything was fine for a while and, of course, it threw the same code. Here I am at about 5,000 miles and I've thrown the same tire sensor code 5 times. Out of frustration, I decided to set it back to stock and just deal with the speedometer error. Well...now the speedometer is dead accurate! Um...how? Thoughts?

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How are you determining if it is “dead accurate”

If it is GPS it needs to be a good one. If your using an APP on your phone i would not trust their accuracy

Sent while firmly grasping my redline lubed RAM [emoji231]

Two ways--my wife's truck paced me and the local "bust you cam" both jived on my speed.

alejes02 said:

Two ways--my wife's truck paced me and the local "bust you cam" both jived on my speed.

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Well that is strange[emoji848]

Frankly it makes no sense at all. Running 35’s and your speed is correct without adjusting the speedometer...

You 100% sure it is back to the stock settings?


Sent while firmly grasping my redline lubed RAM [emoji231]

Tire sensor shouldn't have anything to do with speedometer or calibrating for the speedo.

So I learned something when I was playing around with this on a different truck I used to own.

It came stock with 265/75/16's, when I bought it it had 315/75/16's. Bad combination, they rubbed whenever I tried to turn tight, and my speedo was off 2-3 mph, but they looked cool. When I needed new tires I backed it down to 285/75/16's.

I also picked up a tuner around that same time, so I figured I'd adjust the tire size in the computer to compensate for the larger tires.

This screwed it up even worse, and I was off a good 5 mph with the 'corrected' tire size.

I set the computer back to the 'stock' size...and my speedo was almost dead-nuts.

So then I did some thinking and let my engineering brain stew on it...and I realized it all comes down to how much compressibility there is in your tires. See, tires have their 'actual, unloaded size' (in your case, 35"), and then they have their 'effective radius,' which is how tall they are when a vertical load is applied. In my case at the time, I had gone to a softer, all-terrain tire that had the same effective radius as the stock size and type of tire. I'm betting that's what is causing your 35's to give you an accurate speed reading with the stock tire programming.

Good theory but there is a perceivable difference in the ride height.

Realizing that tire size and speedometer have zero to do with TPMS/tire sensors... vehicle manufacturers usually have their speedos set up to be slightly optimistic and show a slightly higher speed than actual. Going from a 33 to a 35 tire is about 4% difference (2 mph difference at 50mph). This could explain why speedo seems to be "correct" even after the new tires.

Firetruck41 said:

Tire sensor shouldn't have anything to do with speedometer or calibrating for the speedo.

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Not sure if he means the sensor in the tire. I think he’s referring to the actual speed sensor at the wheel


Sent while firmly grasping my redline lubed RAM [emoji231]

alejes02 said:

Good theory but there is a perceivable difference in the ride height.

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It's...actually not a theory, it's proven out by the math and real world testing (by others, not me).

Also, I left out that I never put stock size tires back on that truck, so I can't compare what the 'stock setting' did on 'stock tires.' But as Firetruck41 points out, OEMs do program their speed sensors and speedos to show a higher speed than actual, so I'm presuming I would have seen a lower speed per GPS than my speedo showed. My truck had a noticeable difference in ride height as well. It still doesn't change the fact that combining what OEMs do in programming and tires compress down to an effective radius that is smaller than their actual unloaded size means that going up only a little in size means you really don't need to mess with your speedo calibration.

2003F350 said:

It's...actually not a theory, it's proven out by the math and real world testing (by others, not me).

Also, I left out that I never put stock size tires back on that truck, so I can't compare what the 'stock setting' did on 'stock tires.' But as Firetruck41 points out, OEMs do program their speed sensors and speedos to show a higher speed than actual, so I'm presuming I would have seen a lower speed per GPS than my speedo showed. My truck had a noticeable difference in ride height as well. It still doesn't change the fact that combining what OEMs do in programming and tires compress down to an effective radius that is smaller than their actual unloaded size means that going up only a little in size means you really don't need to mess with your speedo calibration.

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Well, it is wrong. The tire has a fixed circumference. The circumference changes very little with air pressure. It changes 0 with load. Don't believe me. Look at a track vehicle. The fact that the tracks are not round doesn't change the distance traveled for one revolution of said tracks.

boxofrokx said:

Well, it is wrong. The tire has a fixed circumference. The circumference changes very little with air pressure. It changes 0 with load. Don't believe me. Look at a track vehicle. The fact that the tracks are not round doesn't change the distance traveled for one revolution of said tracks.

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Actually you are correct but also wrong. The rolling diameter is always less than the unloaded diameter. The difference is not contradictory because the the tread flexes as it goes through the rotation. Choose any point on the tire and in a 360degree rotation it will be outside the circle determined by 2piR where R is the loaded radius or the distance between the center of the axle and the ground except when it it is directly under the axle.

Tall tires (70-80 series) have more sidewall flex than lower profile (40-50 series) tires so they have a greater difference between static and loaded radius. Low profile tires are usually within 2-3% while 75 series are around 5%.

So it is quite possible for a taller tire with softer sideways (think flotation sizes) to have the same rolling diameter as a shorter and stiffer sidewall.


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boxofrokx said:

Well, it is wrong. The tire has a fixed circumference. The circumference changes very little with air pressure. It changes 0 with load. Don't believe me. Look at a track vehicle. The fact that the tracks are not round doesn't change the distance traveled for one revolution of said tracks.

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Track vehicles are not the same as air-filled tires. This is an apples-to-oranges comparison.

The circumference can change a LOT based on air pressure - internal or external, also based on tire construction. And if you believe it changes zero with load, then you've never watched a tire get loaded. Loading a tire does, in fact, change its rolling, or effective, radius. So while you're right, it doesn't change the RADIAL speed, it DOES change the linear speed of the overall vehicle.

boxofrokx said:

Well, it is wrong. The tire has a fixed circumference. The circumference changes very little with air pressure. It changes 0 with load. Don't believe me. Look at a track vehicle. The fact that the tracks are not round doesn't change the distance traveled for one revolution of said tracks.

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Like mentioned in the last few posts, the effective circumference does change with air pressure and load. You’re using two dimensional figures as examples. Yes, we can take a circle made of string, and pull and maneuver the string in any shape, and the circumference will always be the same. Now, take a tire, and you have a third dimension- the bulge on the sides of a tire when air is let out or load put on it, and the circumference is changed. An extreme example would be, if you let all the air out of the tire, you would almost have the effective rolling circumference of the rim it’s mounted on.

I didn't quote F350's post were he said

So then I did some thinking and let my engineering brain stew on it...and I realized it all comes down to how much compressibility there is in your tires. See, tires have their 'actual, unloaded size' (in your case, 35"), and then they have their 'effective radius,' which is how tall they are when a vertical load is applied. In my case at the time, I had gone to a softer, all-terrain tire that had the same effective radius as the stock size and type of tire. I'm betting that's what is causing your 35's to give you an accurate speed reading with the stock tire programming.

As mentioned, c=2pir. F350 statement leads to the r in the formula being the distance from the axle cl to the ground. This is in fact wrong and it is wrong for the reasons I stated. Did I base my description one a freshman physics type approach were you disregard minuscule effects on the final answer? Yes. I was trying to get to the point that the circumference is not based on the 'how tall they (tires) are when a load is applied.

Want a apples to apples? Air down 1 tire to 10 psi or so. That wheel will spin at near the same rate (again freshman physics differences) as the aired up tires.

Rolling vs static circumference is really just simple geometry.
Rolling circumference is determined by the distance between axle center line and ground. 2piR is the circumference that is used in revolutions per mile calculation. Since R is less than the unloaded radius the 2piR circumference using the unloaded radius will be greater. This results in the unloaded circumference being outside the circle of the loaded radius which causes the tread to travel a greater distance than the ground distance per revolution.

Try it. Air the tire up and measure how far the vehicle goes in exactly 10-20 revolutions. Drop tire pressure by about 75% and repeat the measurement. You will find the vehicle traveled less distance. It won't be a large distance but it will be measurable.

Update. I should clarify that the linear distance the vehicle travels in one tire revolution is what changes not the tread circumference. A given point on the tread will travel the same distance but it will hit the ground again at a shorter distance as the tire is aired down.

BTW there is a passive TPMS that uses this effect and recognizes that a low tire has a higher angular velocity to cover the same distance.

boxofrokx said:

I didn't quote F350's post were he said

So then I did some thinking and let my engineering brain stew on it...and I realized it all comes down to how much compressibility there is in your tires. See, tires have their 'actual, unloaded size' (in your case, 35"), and then they have their 'effective radius,' which is how tall they are when a vertical load is applied. In my case at the time, I had gone to a softer, all-terrain tire that had the same effective radius as the stock size and type of tire. I'm betting that's what is causing your 35's to give you an accurate speed reading with the stock tire programming.

As mentioned, c=2pir. F350 statement leads to the r in the formula being the distance from the axle cl to the ground. This is in fact wrong and it is wrong for the reasons I stated. Did I base my description one a freshman physics type approach were you disregard minuscule effects on the final answer? Yes. I was trying to get to the point that the circumference is not based on the 'how tall they (tires) are when a load is applied.

Want a apples to apples? Air down 1 tire to 10 psi or so. That wheel will spin at near the same rate (again freshman physics differences) as the aired up tires.

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But to explain the issues OP had in this thread, you CANNOT ignore all outside forces and actions. As the rolling radius of the tire changes based on air pressure, loading, and tire construction (because it DOES), then the linear speed of the vehicle WILL change compared to the radial speed of the tire. Unless you are using solid tires, then the rolling radius is what you go by.

I hate sounding cliche or like a know-it-all, but I have an engineering degree in automotive powertrain design. I spent YEARS studying this stuff. This is a critical part of designing ANY automotive powertrain, from the engine to the tires. Sometimes it takes me some time to remember these details, because I no longer work in the automotive industry.

lpennock said:

Rolling vs static circumference is really just simple geometry.
Rolling circumference is determined by the distance between axle center line and ground. 2piR is the circumference that is used in revolutions per mile calculation. Since R is less than the unloaded radius the 2piR circumference using the unloaded radius will be greater. This results in the unloaded circumference being outside the circle of the loaded radius which causes the tread to travel a greater distance than the ground distance per revolution.

Try it. Air the tire up and measure how far the vehicle goes in exactly 10-20 revolutions. Drop tire pressure by about 75% and repeat the measurement. You will find the vehicle traveled less distance. It won't be a large distance but it will be measurable.

Update. I should clarify that the linear distance the vehicle travels in one tire revolution is what changes not the tread circumference. A given point on the tread will travel the same distance but it will hit the ground again at a shorter distance as the tire is aired down.

BTW there is a passive TPMS that uses this effect and recognizes that a low tire has a higher angular velocity to cover the same distance.

Click to expand...

Thank you, sometimes I can't get my examples into words, just equations.