Actual tire diameter

Fitting oversize tires, raising and lowering, suspension modifications...

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kenfyoozed
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Actual tire diameter

Post by kenfyoozed »

When a tire is listed as a size, then it gives the actual diameter, is this diamater while on the rim and truck or off? The reason I ask is I know a 235/75/r15 should be about 29" un diameter, while actual diamter is more like 28.4". I measured my 235/75r15 pn my truck with the proper amount of air pressure and they measured 27 3/8". thats a fare cry from the 29 they should be. Is this the genereral rule or do I have really small 235's? They are the dakota pepboys brand.
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Re: Actual tire diameter

Post by HenryJ »

Some sizes although marked the same can run smaller. Import cases tend to run small.

I do not know how the set the specification for diameter and under what conditions. I do not know all the specifics, but can offer a little insight.

To calculate tire size:
235 is the tread width metric measurement. That is 9.25 inches.

75 is the percentage of tread width that the sidewall measures. .75 of 9.25 is 6.9

To calculate the diameter: Sidewall+Sidewall+Wheel=Diameter 6.9+6.9+15=28.8

So in this case the tire starts its new life 28.8". The tread depth is usually around 16/32" or .5" , so 1" of that diameter wears off during the lifespan of the tire. That means at the end of life the diameter will be 27.8"

You say yours measured 28.4" That would suggest they are at 60% tread. .4" used is a subtraction of 40%.

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Re: Actual tire diameter

Post by kenfyoozed »

Sorry HJ, the 28.4 was used as an example. My tires actual measure is 27.4.

My tires have about 20k miles on them and they are 50k mile tires. So they have gone trough less than have the tread.

If my tires measure 27.4 and add the used tread of .5" , that gives me a new tire measurement of 27.9.

Is this normal to be that far out from the mathmatical measurement of 28.8?

Tires are dakota a/t's from pepboys. they are supposed to be made by cooper I believe.

Just trying to figure out what tire size I can run after the MML minus skid cuts.
I have some more questions but I'll reasearch somemore first.
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Re: Actual tire diameter

Post by HenryJ »

kenfyoozed wrote:Sorry HJ, the 28.4 was used as an example. My tires actual measure is 27.4.
My mistake I read it wrong :oops:
If my tires measure 27.4 and add the used tread of .5" , that gives me a new tire measurement of 27.9.

Is this normal to be that far out from the mathmatical measurement of 28.8?..Tires are dakota a/t's from pepboys. they are supposed to be made by cooper I believe.
Sure. Import cases tend to run small. Coopers are an import case and do run rather small. Mastercraft, and several others all use the same case.
Just trying to figure out what tire size I can run after the MML minus skid cuts.
You will be limited by the small fender openings to no more than a small 31" tires. Pay close attention to the shoulder design. A rounded shoulder will offer more clearance.

Each truck can be a little different. The only way to know for sure it to take your own measurements. Pu the front tire up on a ramp or curb to max the upward movement , or close. Turn the wheel to where the shoulder of the tire is closest to the rear of the front fender. Measure.
Now you can use a square to see what difference you will have if you do a body lift. Use a straight edge to add a larger diameter tire.

I ran a set of Geolandar MT that cleared very well. They had a rounded shoulder and ran very small. The BFG TAKO tend to run larger still just below spec. TSL, Baja Radials and some others tend to run to size or even larger.

It is not an exact science. Do your best to plan and hope it works for you.

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Re: Actual tire diameter

Post by kenfyoozed »

I figured without the cuts it would be either a small 31x10.5 or a large or true to size 30x9.5. A true 30 would give me an inch more lift, right? or from where I am at right now 1.25" roughly, then add the bl and tbcrank and the truck should sit 4.75 inches higher roughly, correct.

It sure would be nice to run those 33's, but afraid it might kill my power and I am scared of making those cuts and screwing something up.
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Re: Actual tire diameter

Post by F9K9 »

kenfyoozed wrote:I figured without the cuts it would be either a small 31x10.5 or a large or true to size 30x9.5. A true 30 would give me an inch more lift, right?.....
Going from a 29 to 31" tire is 1" of lift.
kenfyoozed wrote:.......then add the bl and tbcrank and the truck should sit 4.75 inches higher roughly, correct........
The BL is 2". 1.5" is just about all you can hope for but, I'd expect less. You still need to raise the back by either a spring pack or shackles.
kenfyoozed wrote:.......It sure would be nice to run those 33's, but afraid it might kill my power and I am scared of making those cuts and screwing something up.
I didn't even know what a sawzall was before I joined the forum. You'll do fine once you make the first cut. :thumb:
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Re: Actual tire diameter

Post by kenfyoozed »

Hj, did you have the skids when you ran the geolanders?

F9k9, i have all the tools to do and have used them before just not on my truck :shock:

I thought of doing the skids and then rhino line the fender and the lower part of the truck but dont know how that would look.

I didnt have a curb but i did have a 6x6 block i drove up on. I had about an inch all around except at the frame. I couldnt distort myself in there but i had about a fingers with clearance to the frame. So I am sure anything will rub on the frame so will probably have to add to the steering stops as mention on here.
Last edited by kenfyoozed on Sun May 09, 2010 9:19 am, edited 1 time in total.
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Re: Actual tire diameter

Post by HenryJ »

kenfyoozed wrote:Hj, did you have the skids when you ran the geolanders?
No. T-bar adjustment and body lift.
I thought of doing the skids and then rhino line the fender and the lower part of the truck but dont know how that would look.
Image
I plan to do it someday. BTW, those are the Geolandars with Bushwacker extend-a-fender flares.

Browse the readers rides. bwenny247 has lined rockers and over the wheels.

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Re: Actual tire diameter

Post by kenfyoozed »

Got any other pics with the geo's and bushwackers? I really like that look since my truck is that same color and same wheels.

The geolanders measure 30.6 and you had no rubbing issues ? Or had you already added the spacers?
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Re: Actual tire diameter

Post by HenryJ »

Do a search and use me as the author. You will find lots. The Geolandar MT fit better than the AT+II due to the shoulder design being rounded on the MT. Both have been discontinued , so don't plan on either being an option. You can use them as examples and look for something comparable. Good luck with that!

Here is a start: Extend-a-fender Flares

Keep in mind that these flares are not "plug-n-play" They are too small for the rear fenders and have lots of trimming to fit the Isuzu bed. Once done you will have made the wheel well a smaller opening. Measure it. It is already tight at 31".

These reasons and more are why we came up with the fender cuts for the Skidz install. They make a world of difference.

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Re: Actual tire diameter

Post by kenfyoozed »

Tire rack list geolander mt plus

http://www.tirerack.com/tires/tires.jsp ... mpare1=yes


I didnt know if these were the same or comparable?
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Re: Actual tire diameter

Post by HenryJ »

They look about the same: Estimated Availability: Fewer than 2

They are directional, so rotation is a problem.

Be prepared, they wear very quickly. I loved the performance, but they were short lived.

"Speed doesn't kill, suddenly becoming stationary does." - Richard Hammond
"Speed is just a matter of Money - How fast do YOU want to go?"-Mechanic from Mad Max-
If at first you don't succeed - Don't take up Skydiving!
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Re: Actual tire diameter

Post by kenfyoozed »

thanks on the heads up about the direction. Ididnt even thank of that.