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Size Difference in 24.5 Semi Tires?
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Eddy in TN
Posted 2/16/2012 07:39 (#2231733)
Subject: Size Difference in 24.5 Semi Tires?



Pulaski, TN
What is the size differences in a 11R24.5 and a 28575R24.5? Found some cheap of the last size and truck has the 11R24.5 on it now. These will be going on the drive axles. Not replacing all tires. Thanks!!
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eddiedry
Posted 2/16/2012 08:28 (#2231824 - in reply to #2231733)
Subject: Re: Size Difference in 24.5 Semi Tires?



Wheatley, Arkansas
A 285 is a lo pro design. A 285/75R24.5 would match up pretty good with a 11R22.5 tire in a side by side. I don't think you would want 11R24.5's on the same axle or tandem as a 285/75R24.5.
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cotncrzy
Posted 2/16/2012 08:34 (#2231839 - in reply to #2231733)
Subject: RE: Size Difference in 24.5 Semi Tires?



PROSPECT, TENNESSEE
Eddy, I am not a tire man but I think the tire would be wider, and shorter (low pro). You know anything about corn meters? Pulling mine off today, going through them.
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durallymax
Posted 2/16/2012 08:35 (#2231842 - in reply to #2231733)
Subject: Re: Size Difference in 24.5 Semi Tires?


Wi

Truck tires are kinda a pain to figure out if you dont have the formulas. I don't know why they keep standard sizes and dont just label them all in metric like automotive has done for the most part as well as ag.

11R24.5 means the tire is a standard profile tubeless tire. 11 is the width in inches. Standard tubeless tires have an aspect ratio of 88% which means an 11R24.5 would be 43.86" tall overall (depending on tread depth) with a radius of 21.93". (11"x0.88=9.68"x2=19.36"+24.5"=43.86"/2=21.93")

285/75R24.5 tires would be 41.33" Overall with a radius of 20.676" (285mmx0.75=213.75mmx2=427.5mm/25.4=16.83"+24.5"=41.33"/2=20.66")

So your overall gain is 2.53" in diameter or 1.27" radius or about a 6% gain.

How does this equate to RPMs? Heres the formulas

Tire diameter x pi (3.1415.........)=Circumference
Mile (5,280')x 12 (to convert to inches)= 63,360" in one mile.
63,360/Circumference=Tire RPM

Sooo
pi*41.33"=129.84"
pi*43.86"=137.79"

63,360"/129.84"=460RPM
63,360"/137.79"=488RPM

Alternatively if you already know your tires diameter and its RPM and know the other tires diameter you can figure the difference percentage wise and apply that to the RPM you know. You know the difference is 6% so if going up multiply by 1.06 if going down multiply by 0.94.

1.06x460=488
0.94x488=460

on a side not 11.00-20 for example is a tube type standard profile tire, these have an aspect ratio of 98%.

If you go to these tires you will need them on all 8 drives you cannot mix and match, technically they want you to keep each pair of tires within 1/8" of each other.

Hope this helps you in the future with all tires.



Edited by durallymax 2/16/2012 09:19
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Eddy in TN
Posted 2/16/2012 08:45 (#2231861 - in reply to #2231842)
Subject: Re: Size Difference in 24.5 Semi Tires?



Pulaski, TN
I copying this formula and putting it in my truck file!!! Thanks!! Knew the cheap tires were too good to be true!!!
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Eddy in TN
Posted 2/16/2012 08:46 (#2231862 - in reply to #2231839)
Subject: Re: Size Difference in 24.5 Semi Tires?



Pulaski, TN
You are correct on the tires. I would take those corn meters to a test stand and see about them. That's what I'm going to do!!
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cotncrzy
Posted 2/16/2012 08:49 (#2231871 - in reply to #2231862)
Subject: Re: Size Difference in 24.5 Semi Tires?



PROSPECT, TENNESSEE
where???
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ekeller2
Posted 2/16/2012 09:09 (#2231919 - in reply to #2231733)
Subject: Re: Size Difference in 24.5 Semi Tires?


So. IL
We had a trailer one time that had 22.5 low profile on one axle, and 22.5 tall on the second axle. Everyone told us it would not work but we never had and problems with it.
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durallymax
Posted 2/16/2012 09:18 (#2231940 - in reply to #2231839)
Subject: RE: Size Difference in 24.5 Semi Tires?


Wi

cotncrzy - 2/16/2012 07:34 Eddy, I am not a tire man but I think the tire would be wider, and shorter (low pro).  .


11x25.4=279.4mm/285mm=0.98

5.6mm or 2% wider for the low pro.

11x0.88=9.68x2=19.36+24.5=43.86/2=21.93" Radius
285x.075=213.75x2=427mm/25.4=16.8+24.5/2=20.66" Radius So 1.27" Difference in Radius. Or 6% 

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durallymax
Posted 2/16/2012 09:29 (#2231959 - in reply to #2231861)
Subject: Re: Size Difference in 24.5 Semi Tires?


Wi

Eddy in TN - 2/16/2012 07:45 I copying this formula and putting it in my truck file!!! Thanks!! Knew the cheap tires were too good to be true!!!


Not necessarily too cheap to be true. You have to weigh it out. Low pro 24.5s are lighter, and have slightly less rolling resistance. However the gain from those would be lost in engine speed maybe. Here are the formulas for calculating that.

336.1355/Tire Diameter*Rear Axle Ratio*Desired Speed= Engine RPM in Direct (1:1 ratio in transmission). If you have an overdrive transmission multiply the RPM in direct by the overdrive ratio. Most Eatons are around 0.73-0.75. 

Here is an example assuming a 3.70 Rear end with tall 24.5s (11R24.5)

336.1355/43.86=7.66
7.66x3.70=28.34
28.34x65=1,842
1,842x0.74=1,363

You know that the Low-Pro's are 6% Shorter So apply that.

1,363x1.06=1,445

So you gain 82 Engine RPM. Only you will know what this might do for your fuel economy. For most farmers, it doesnt matter we never run enough miles.

If you want I could put all of the formulas into an excel spreadsheet. 

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ccjersey
Posted 2/16/2012 10:24 (#2232054 - in reply to #2231733)
Subject: Re: Size Difference in 24.5 Semi Tires?


Faunsdale, AL
Your trailer doesn't have any connection between the axles or side to side on each axle, so as long as you didn't put two next to each other on one side (dualed) you would get along fine with that.
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durallymax
Posted 2/16/2012 11:13 (#2232140 - in reply to #2232054)
Subject: Re: Size Difference in 24.5 Semi Tires?


Wi

ccjersey - 2/16/2012 09:24 Your trailer doesn't have any connection between the axles or side to side on each axle, so as long as you didn't put two next to each other on one side (dualed) you would get along fine with that.


Exactly, Front to rear the suspension will just compensate if even needed (depending on suspension).

Side to side could cause some wear issues

Same hub (duals) would cause the most issues.

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Eddy in TN
Posted 2/16/2012 12:51 (#2232327 - in reply to #2231871)
Subject: Re: Size Difference in 24.5 Semi Tires?



Pulaski, TN
John Deere dealer can do it. There is a place in New Market, AL that used to check them. You can sent them to Shoup. I'll do some checking to see where.
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eddiedry
Posted 2/16/2012 16:40 (#2232643 - in reply to #2232140)
Subject: Re: Size Difference in 24.5 Semi Tires?



Wheatley, Arkansas
Front to rear on drive axles will not necessarily compensate for different tire sizes.
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durallymax
Posted 2/16/2012 23:42 (#2233665 - in reply to #2231733)
Subject: Re: Size Difference in 24.5 Semi Tires?


Wi
all depends on the suspension type, style and setup. Some do, some dont some can be converted to.
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