# Taller tires - computer recalibration

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• posted on November 27, 2003, 9:23 am
wrote:

Mike, determining tire height with radius suits better for those who want more academic conversation and more "convincing" formula. They have the joy of multiplying the radius by two to get what we know without multiplying, "how tall the tire is", diameter.
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Jukka

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• posted on November 29, 2003, 2:49 am
Not bad post has lasted 5 days so far.

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• posted on November 29, 2003, 7:11 pm
snipped-for-privacy@thanks.net says...

75% percent of 235 is more than 75% of 215.
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• posted on December 2, 2003, 2:49 am
I work at a tire shop and can say that for Example, you have a 235/75R15, and a 215/75R15, that the 235 will be TALLER!!! It would in fact be quite a bit taller, by a couple Inches. Your moving up 2 sizes, since you skipped a 225/75R15. If you had a 215/75/R15 before and wanted to keep the SAME Height but get a little wider, then a 225/70R15 would be the tire to get. Or Wider yet with a 235/65 or 60 R15 Depending on the Brand of tire since they do very from different Manufactures.
I have P275/45R20 on my own truck. The Height is quite close to the Stock Chevy P245/75R16" wheels that were on in. Much Wider tire but not a whole lot of sidewall. Ignoring the Wheel Diameter or either 16" or 20", you can see that the first number is going up and the Second Number is going down. So while the height is the same, the tire is wider by quite a bit. If I needed to replace the stock 245 tires, but want to keep the height but a little wider, again I'd go with a P255/70R16. The height the same, the Width a bit wider.
If someone says there want to go up from a 215 to a 235 and don't give the second # then I and anyone else will assume that the second number is the SAME. In which case the tire WILL BE A LOT TALLER! A couple inches is a lot. Your jumping up 2 tire sizes.

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<%-name%>
• posted on December 5, 2003, 5:00 am
Seeing as you are referring to metric sizes, and those numbers are of course in millimeters. Measured from widest point to widest point, usaully the sidewall bulge. Using this example below, the tire will be 15 mm taller, or 1.5 cm, or .59"

on.
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• posted on December 5, 2003, 6:06 pm
You are mistaken.
On the tires I was referring to, the BFG line, there is a 1.2" difference in height between a P215x75 and a P235x75 tire of the same make and type.
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's
Chevguy wrote:

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• posted on December 5, 2003, 9:31 pm
You say 6, I say a half dozen. It is 1.2" diameter, or .6" radius difference. I had said .59". Sorry I was off.

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• posted on December 5, 2003, 10:11 pm
No, I am saying the word 'taller'.
.59 difference in radius is ALMOST the same as 1.2" diameter taller.
It is 'not' .59" 'taller' at all.
No ifs, ands, buts or maybes, the tire is 1.2" taller period!
I don't get why you folks are tossing in an unrelated term when trying to figure how much 'taller' a tire is.
The manufacturers give the height of the tires to compare on their charts.
You need this height difference 'that they give you'! to figure out the circumference difference so you can figure out the speedo difference.
THEY DON'T GIVE A RADIUS MEASUREMENT!
Man it isn't rocket science, it is grade 6 math.
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's
Joe Poitras wrote:

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<%-name%>
• posted on December 5, 2003, 11:06 pm
Then you should know that you figure out circumference using the radius, not diameter.

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<%-name%>
• posted on December 6, 2003, 12:47 am
says...

You figure out height using the top, not the middle.
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• posted on December 6, 2003, 1:16 am
How do you figure that?
I run BFG tires and have checked out lots of other makers and 'all' of them give the height or diameter in their charts.
Reality time here.
So to figure out the circumference all you have to do is multiply the diameter by PI or 3.14.
What could be more simple?
I 'Really' don't understand why folks want to take the diameter that the tire makers give you, divide it in half, then use Rx2xPI.
Makes no sense in my mind.
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's
Chevguy wrote:

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• posted on December 6, 2003, 1:35 am

because most calculations for speed vs size are based upon 'loaded RADIUS', the distance from the center of the axle to the pavement, with the tires at correct pressure.
which is different from diameter or 'radius' by a good margin (measure it yourself), either with a tape measure, or by calculation speed vs RPM via any of the available formulas, using half the diameter in your calculations, then check it with a GPS
you'll find quite a difference between what you 'should have' and what you actually have

yeah, well..............
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• posted on December 6, 2003, 6:27 pm
Gary Glaenzer wrote:

The question was about a speedo recalibration.
If the OP goes to the makers tire charts and gets the given diameter of the two tires in question and uses those diameters to 'compare' the % difference between them, the speedo should be off almost exactly that same percent.
If they want to try and figure the rolling radius of both tires and compare them for percent differences, they will be damn near be the same percent difference as taking the given makers numbers and figuring it that way.
Is that just too simple for folks?
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's
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• posted on December 6, 2003, 4:50 am
And I was merely pointing out that jumping 20 mm in width is not a couple of inches of height as stated earlier.
Regardless if you say .6" extra technical ride height or 1.6" overall height. You will not be crushing cars with them.

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• posted on December 6, 2003, 9:03 pm
The height difference between the two tires I used for comparison is 1.2" period. In my mind, that is a 'lot' taller.
I don't give a shit if one is wider or narrower, that doesn't help the original poster who wants to calibrate his speedo.
He don't care how much clearance extra it will give him, he wants to know how much the speedometer is going to be off.
The formula for circumference is PI X D.
The tire makers give you D.
They do not give you R so you can double it to find D, they give you D. Simple eh?
Forget the fancy metric P ratios, the OP could care less about that, he just wants to know the percent difference between the tires travel or circumference so he knows how much percent the speedo will be off.
Mike
Chevguy wrote:

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• posted on December 6, 2003, 9:32 pm

speedo
speedo.
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<%-name%>
• posted on December 6, 2003, 10:05 pm
LOL!
Good one.
Mike
Chevguy wrote:

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• posted on December 6, 2003, 3:37 am
The circumference of a circle can be calculated using either the radius or the diameter.
C = pi * Diameter
or
C = 2 * pi * Radius
If the ire is approximately 32 inches tall (265/75R16) then:
C = 3.14 x 32 or 100.48 inches traveled for a full revolution
..just my 2 pi worth.
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