Did my tire size actually change

[planojeeper]

I have a 2021 Texas Trail Edition and my mud terrain tires were ready to be replaced. The original tires were BFGoodrich KM2 32–Inch LT255/75R17C which shows to be 32.09". I wanted to get something a little bigger but be able to keep everything else stock as I haven't started modding yet and went with Falken Wildpeak A/T4W LT255/80R17 which based on specs would be 33.1" tall.

Apologies for the noob question but when I measure the tires from the ground up, they are 32 inches. I was going to reprogram the tire size with JScan due to thinking the tires were different height based on spec but at the moment, they appear to be the same height. JScan shows the setting is already set for 32.09". Is this normal or is there something I'm missing. Did I get the wrong size tire thinking that I was getting ~33" tire with LT255/80R17? Or is it something like the tire pressure so not getting to 33”?

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[Wheelin98TJ]

Tire sizes vary quite a bit. I don’t think it’s anything to be concerned about. Just set the speedo right and run them.

 

[Lunentucker]

Measure ground to center and multiply X2

 

[planojeeper]

Measure ground to center and multiply X2

Interesting, looks like the deflection from the weight makes just enough difference. When measuring from center to top and multiple by 2, I get 33. When measuring to the bottom times 2, it's 32.

 

[KevinM60]

I have a 2021 Texas Trail Edition and my mud terrain tires were ready to be replaced. The original tires were BFGoodrich KM2 32–Inch LT255/75R17C which shows to be 32.09". I wanted to get something a little bigger but be able to keep everything else stock as I haven't started modding yet and went with Falken Wildpeak A/T4W LT255/80R17 which based on specs would be 33.1" tall.

Apologies for the noob question but when I measure the tires from the ground up, they are 32 inches. I was going to reprogram the tire size with JScan due to thinking the tires were different height based on spec but at the moment, they appear to be the same height. JScan shows the setting is already set for 32.09". Is this normal or is there something I'm missing. Did I get the wrong size tire thinking that I was getting ~33" tire with LT255/80R17? Or is it something like the tire pressure so not getting to 33”?

Unless you measured them at 32.09 your original tires might not have been that height. From what I've seen the actual sizes tend to run a little smaller than spec.

 

[Lunentucker]

Interesting, looks like the deflection from the weight makes just enough difference. When measuring from center to top and multiple by 2, I get 33. When measuring to the bottom times 2, it's 32.

Now the final trick. Measure it at 60 MPH.

 

[ShadowsPapa]

Interesting, looks like the deflection from the weight makes just enough difference. When measuring from center to top and multiple by 2, I get 33. When measuring to the bottom times 2, it's 32.

Dynamic tire circumference. Not static specified diameter. The diameter they give is laying flat, uninflated, not mounted.
The final number will vary with a number of factors.

I get it close, then drive it and measure the miles and tenths driven compared to a GPS or a GPS app on my phone.
I set my trip odometer to 0, start up a GPS or GPS app, and drive, after driving a while, it should match miles on trip odo which measures to tenths of a mile vs. the GPS or GPS app.
In my case, both came out at 17.1 miles so i was good to go after 2 tries.
I never used the internet tire size specs.

What you measure and what it ends up being will vary because it's actually based on rolling or dynamic tire circumference and not tire diameter itself.
Circumference as it rolls determines how far you go per tire rotation. It's even in the system like this:

Dynamic Tire Circumference (All or Rear) (0-65534 mm):

But tazer or apps will do calculations for you and that's why the first try isn't always right on.

 

[DailyMoparGuy]

On both occasions of me programming tire size (285/70/17 Falkens & 315/70/17 Kendas ), I put the diameter listed by the manufacturer. Both times gave me accurate speedo readings.

Maybe luck, but I recommend starting with the listed diameter from the manufacturer. As mentioned, rolling/dynamic diameter is likely greater than static diameter (or circumference).

 

[planojeeper]

Wow, thank you for the awesome responses. Totally makes sense. I’ll try comparing miles driven with gps and odometer along with speed to see how it looks and adjust.

There is a lot more to it than I thought.

 

[Mr._Bill]

Wow, thank you for the awesome responses. Totally makes sense. I’ll try comparing miles driven with gps and odometer along with speed to see how it looks and adjust.

There is a lot more to it than I thought.

When you do the comparison testing, do not connect the phone to the head unit. This keeps the readings separated. If you have one available, use a separate GPS device for accuracy, or a tablet with a GPS app, to compare against the odometer.

 

[Escape.idiocracy]

https://apps.apple.com/us/app/speedometer-simple/id939507215

 

[dayusmc]

I do it the simple way. I put in as close to what I measure the tire at. Then I drive the truck on the highway at 65 and have my wife look at her phone's gps (her phone is not connected to my truck). Based off of that I adjust tire size up or down. I find it much easier if there are two people doing it. I watch the ride and the truck's speedometer and she watches her phone and tells me what it says... I like to keep it simple, I am a Marine.
I am sure Shadowspapa's way is the correct way, but it is to complicated for me. I hate doing math....

 

[Russ2023]

Interesting, looks like the deflection from the weight makes just enough difference. When measuring from center to top and multiple by 2, I get 33. When measuring to the bottom times 2, it's 32.

That’s because when you measure from center to the ground you’re measuring the tire under load and that would only rarely be accurate

 

[bleda2002]

That’s because when you measure from center to the ground you’re measuring the tire under load and that would only rarely be accurate

This is the answer. Different tires squish more (BFG which people call small) some less (Kenda's for example), but the actual circumference is what matters. Start with the stated actual size, which for pmetric would be the 33.1, then adjust up or down by gps.

 

[planojeeper]

Thanks everyone for your input. I used the speedometer app and compared to the actual speedometer, it was off as expected and by the amounts shown using https://tiresize.com/comparison/.

Used jscan to adjust to 33" and now the speedometer matches the speedometer app. Looks like I'm all set. Tire squish is a thing and impacts if going based on the ground measurement vs tire spec. In my case not a huge impact (but noticeable) but if anyone goes to a larger tire, would have a bigger impact.

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