Taller tires - computer recalibration

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"

Any change from 'factory' will alter alignment..

-- History is only the past if we choose to do nothing about it..

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.

See:

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Chevguy wrote:

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.

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:

Then you should know that you figure out circumference using the radius, not diameter.

You figure out height using the top, not the middle.

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:

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..............

"Randy Howard" wrote

Let's put it this way: the opinion that says raising or lowering front ride height by adjusting the torsion bars will "not" change the alignment is wrong.

Inversely....the opinion that says that raising or lowering front ride height by adjusting the torsion bars "will" change the alignment is correct.

Ian

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.

And I was merely pointing out that jumping 20 mm in width is not a couple of inches of height as stated earlier.

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

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:

What's more simple, is that they will tell you #rev's per distance for different sizes. Using the BF site, a vehicle calibrated to 215 75 15 using 235 75 15 will be out 4.18%

> > > > > >

message

LOL!

Good one.

Mike

Chevguy wrote:

impossible.

================================= No big deal, your speedo will be off a little, and odo too. big woop. :) Hell when it comes time to sell it, it will show less miles than it actually has! :) Or you could buy a programmer for about $400 to change it.