What size tires do you really have?

bobthetj03

bobthetj03

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We see a lot of questions about tire size and will X" tire fit, and what should I re-gear to with X" tire. Thought I'd show a little demonstration of my "actual" tire diameter. So, according to BFG's website, my 33X1050X15 KM2's should have a diameter of 32.8", un mounted of course. This is with 18/32" tread depth.
My 3800lb. jeep with 27psi in the tires, and 14/32" of tread depth measure in at just under 32". Sorry the last pic is a bit blurry, but you get the gist. Just food for thought.

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Rob5589 said:
Exactly the reason to use actual tire diameter, not what the sidewall reads, when deciding on gear changes and such.
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Wouldn't the actual rolling radius be from the center of the wheel to the ground. That would take into account the "squish" associated with the contact patch. It's not much, but it's there.
 
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StG58 said:
Wouldn't the actual rolling radius be from the center of the wheel to the ground. That would take into account the "squish" associated with the contact patch. It's not much, but it's there.
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This would be important to get the true height of the axle. But in my mind, as far as fender clearance and gearing goes, this would actually give you too small of a measurement. Although we're probably talking less than 1/2" difference and it could easily be accounted for if you took the measurement like that. But as far as clearance goes, the part of the tire that's gonna hit your fender isn't squished down; and as far as gearing goes, the engine doesn't care that the side wall is bulging, it's still gotta turn xx" of tire.

I'm probably being too detail oriented here, because it's not like you can choose between 4.88 gears and 4.95 gears anyway. Just wanted to play devils advocate more than anything:devil-flip:
 
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Obviously there are some variables in my measuring technique, but just wanted to share anyway.
 
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bobthetj03 said:
Obviously there are some variables in my measuring technique, but just wanted to share anyway.
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Absolutely. Like I said, I was just playing devils advocate and being too detailed oriented...probably because I'm getting ready for my first set of board exams right now and I have to focus on even the smallest details.

But it's an interesting discussion and I don't think a lot of people realize their tires aren't as tall as the manufacturer said. My friend has 35" BFG AT's on his JK...they're only a hair taller than my 33" MTR's though. Speaking of, I should measure mine and add to the conversation.
 
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If I think about it tomorrow, I'll measure my 31" KO2'S and then run them up to the pressure on the sidewall, and then down to 8 pounds or so. Wonder how much difference that'll make?
 
I just happened to measure my cooper 33s today because I felt like they "looked" smaller than other Jeeps with 33s. They measure at barely over 31 inches with 26psi and they are not new. I know I cant run 35s with my dana 35, But would it be possible to run 35s if actual measurement was 34 inches?
Maybe Im just looking for a loophole in the argument against 35s on a weak axle LOL.
 
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It would be an interesting exercise to measure mine out too. I know they really touted the rubicon when it was introduced because you could finally get a 31 x 10.5 on a factory wrangler. The stock size is 245/75r16, which when you convert that to floatation size, equals 30.5" by 9.5."
 
Adam mendel said:
I just happened to measure my cooper 33s today because I felt like they "looked" smaller than other Jeeps with 33s. They measure at barely over 31 inches with 26psi and they are not new. I know I cant run 35s with my dana 35, But would it be possible to run 35s if actual measurement was 34 inches?
Maybe Im just looking for a loophole in the argument against 35s on a weak axle LOL.
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With a Super 35 kit you could. Axle strength is based on the listed tire size, not actual measurement. So no, your loophole doesn't really work :D
 
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StG58 said:
If I think about it tomorrow, I'll measure my 31" KO2'S and then run them up to the pressure on the sidewall, and then down to 8 pounds or so. Wonder how much difference that'll make?
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I went out with my trusty level, tape measure and Viair pump and did a little experiment. The TJ weighs in at 3,300 pounds. The tires are BFG KO2's in 31x10.50R15 with less than 10,000 miles on them. BFG says they are 30-1/2" in diameter mounted on 8" rims. BFG further states that they make 682 revolutions / mile. These are mounted on 7" wide Grizzlies. The results:
At 8 pounds pressure they measure 29-1/4" from the top of the tire to the ground.
At 28 pounds, the pressure that I run them at, they measure out to 29-5/8" in diameter.
At 50 pounds, they measure out at 30" in diameter.
The spare tire, with no load of course, measures out at 30-1/8" at 28 pounds, and I'm guessing it would go to BFG's 30-1/2 at 50 pounds.

The rolling radius at 28 pounds looks to be about 14-7/8".

I screwed up my revolutions per mile calculation at the various pressures, so I'll add those in later.
 
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Thanks for adding your results StG58. I'm thinking we can pretty much figure that whatever size tire you run, subtract 1" from the advertised size and that's what you really have.
 
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bobthetj03 said:
Thanks for adding your results StG58. I'm thinking we can pretty much figure that whatever size tire you run, subtract 1" from the advertised size and that's what you really have.
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Pretty much, it seems. I was surprised that tire pressure didn't make more of a difference. I was also surprised that you had to go to max sidewall pressure, unloaded, to get the manufacturer's advertised size.
 
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