Tires sizes.

[OB Bronco]

I know this has probably been beat to death, but why do tire manufactures put measurements on their tires, when the measurements aren't even close. Example: BFGoodrich Mud 35" tire that unmounted measures maybe 33.5". So how tall will it actually be when mounted on a 10" rim? I get the manufactures save money, but I just wondered if anyone had ever asked a tire rep? It just seems pointless that at no time will this tire ever be 35". So, why call it that? My rant is over, any input would be appreciated.

Thanks,
Wade

 

[bronconut73]

They measure the width at the bulge so the tread isn't 12.50" either.

I suppose on a 7" rim inflated to capacity (prolly 45 psi+) it may be 35".

Last set of 35" BFG's I bought were a smidge over 34" not mounted.

 

[Apogee]

My 37" BFG KM2's measure about 36.5" tall mounted on 17x9 wheels, but they look kinda skinny with a 13" bulge to bulge, but only ~10" tread width.

 

[OB Bronco]

It just seems odd, but I guess nothing is what it seems. My next thought is when we go to set up our gears are people going off the tire number or actual tire size?

 

[bronconut73]

It just seems odd, but I guess nothing is what it seems. My next thought is when we go to set up our gears are people going off the tire number or actual tire size?

A little of both.

 

[Rustytruck]

Scroll to the bottom to see dimensions.

http://www.bfgoodrichtires.com/tire...50/15/OE/BNW/mud-terrain-t-a-km2/tire-details

 

[OB Bronco]

Thanks for the link, I had done my research. I just stuck in my head, an inch is a measurement just like a pound. Why is it Ok to give me 33.5" tires? I wouldn't except 9/10 of a pound of hamburger meat. I know it is what it is I just wondered if anyone asked a manufacturing rep about it? Any tire company rep on here?

 

[Whoaa]

My 37" BFG KM2's measure about 36.5" tall mounted on 17x9 wheels, but they look kinda skinny with a 13" bulge to bulge, but only ~10" tread width.

Mine are just a little smaller. My 37" BFG KM2's stretched the tape measure just a smidge over 36" mounted on 17 x 9 wheels.

 

[jmangi62]

My 35in. Toyos measure 34" on 17x9 wheels, I have a set of 35in. Nittos on another vehicle, also on 17x9s also measure 34",crazy LOL.

 

[AxlesUp]

my 37" trepador radials measure 36"s on a 17x9 wheel at street pressure with my one ton bronco sitting on them

what i like to measure is ground clearance under the axle. i lost 0"s when i went to the brand new 37s from the worn 39.5 tsl at street pressure. that suprised me.

 

[NYLES]

My 37 Nittos measure 37.5 on 17x9s

 

[JSmall]

My 37 Nittos measure 37.5 on 17x9s

What Nitto's are those?

 

[pbwcr]

Scroll to the bottom to see dimensions.

http://www.bfgoodrichtires.com/tire...50/15/OE/BNW/mud-terrain-t-a-km2/tire-details

======
And of you convert 598 rev/mile to inches you get 33.73" diameter. And in my experience that number is much closer to get your speedo correct.

 

[DirtDonk]

From what I understand about the process, the stated sizes on any tire are the specifications for the actual size of the original molds and fixtures only. The final tire size is whatever it is, which varies by tire model (not individually, but different types and styles) due to shrinkage.
It varies on construction techniques and rubber compounds and types and amounts of material used in the inner belts, and probably temperature variations (if any) used in the vulcanizing process.

Yes, after initial testing they should easily be able to figure out what the tires of a particular model are going to end up as, so should be able to sell a 33" tire, or a 235 tire, that is actually 33 inches tall or 235mm wide. And if they don't want to list a tire as a "33.4" tire, then you would also think they could make the molds a certain amount larger, in order to end up with a tire that is the size they originally expected.
Probably just too much trouble for a non-issue as far as they're concerned.

The ultimate functional diameter of a tire varies so little by wheel width, between 7 and 10 inches anyway, that it's almost not worth including in the specifications. The biggest variation at that point would change more in the rolling radius when mounted on a vehicle.
We've tried the same size tires on different wheels and could not measure any difference. This was off the vehicle though, and not with the weight on it. Not sure what difference that might make, but the rolling radius would vary by vehicle weight anyway, which the tire companies don't have any control over.

The width of the tread changes not one iota as far as I know either. Only the sidewall changed, and even that in extremely small amounts. Not near as much as the width of the wheels would have indicated, because the sidewall just flexes and kind of "rotates" on it's own internal pivot point at the tread.
Only the actual bead of the tire changed any measurable amount, since that was the direct interface with the different wheels, and therefore directly related to the changes.

Paul

 

[AZ73]

======
And of you convert 598 rev/mile to inches you get 33.73" diameter. And in my experience that number is much closer to get your speedo correct.

A tire sitting on the ground isn't 34.8 inches. It's not touching the road at a single point, it's a flat spot. If you took a measurement of the center of the axle to the ground, it would be shorter than the center of the axle to the top of your tire because the radius is shorter due to the weight of the vehicle. If the tire was exactly 34.8, it would take 579.54 revolutions per mile. Because it's compressed by the weight to 33.73 on the side of the tire that's touching the road, it takes the 598 quoted to go a mile. The distance traveled along a straight line (the path of the patch touching the ground) is shorter than the distance traveled along a circle (if the tire was solid and only touching the ground at one point.

 

[OB Bronco]

This is interesting. When I was looking at gearing I kept thinking how the different sizes in the tires affect it?

 

[pbwcr]

What you guys should take from these posts is the info on the sidewall does not tell you the diameter or circumference of the tire. Dirt Paul did not believe this and found out by testing the rim does not affect the tire size.
And AZ73 talks the truth. He did not mention that the circumference of the tire is a fixed parameter and will not change magically. The tire will not create or remove rubber as it travels down the road. What you get is what the tire company made.
Bottom line the tire has a certain circumference and is not affected by the measurements you make for radius or diameter. The tire companies are pretty accurate by publishing the revs per mile. Always use that number if you are changing brands or tire size and want to determine how many revs the engine will make with a tire change.
Some of us care because we want to have an accurate speedo so that is what the process for determining the tire diameter is all about. We care about gearing.
For those who are challenged here is the drill
Go to the tire company tech web site and make a list of the candidate tires and record the “rev/mile” for each tire. The do the math as follows:
Diameter =(5280*12)/(Pi*RPM), Where Pi is the constant 3.1415
Then make your comparison.

 

[BroncoMabry]

What you guys should take from these posts is the info on the sidewall does not tell you the diameter or circumference of the tire. Dirt Paul did not believe this and found out by testing the rim does not affect the tire size.
And AZ73 talks the truth. He did not mention that the circumference of the tire is a fixed parameter and will not change magically. The tire will not create or remove rubber as it travels down the road. What you get is what the tire company made.
Bottom line the tire has a certain circumference and is not affected by the measurements you make for radius or diameter. The tire companies are pretty accurate by publishing the revs per mile. Always use that number if you are changing brands or tire size and want to determine how many revs the engine will make with a tire change.
Some of us care because we want to have an accurate speedo so that is what the process for determining the tire diameter is all about. We care about gearing.
For those who are challenged here is the drill
Go to the tire company tech web site and make a list of the candidate tires and record the “rev/mile” for each tire. The do the math as follows:
Diameter =(5280*12)/(Pi*RPM), Where Pi is the constant 3.1415
Then make your comparison.

Where do I carry the 1 ?