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Old 03-14-2016, 04:52 PM
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Tire Width expectations from different sizes on the same rim

Well, I am adding to all the Tire size posts!!!

But I see a lot of threads regarding diameter and lack there of from the leading tires.

So my question is similar but it is in regard to width. Assuming the same sized rim width from size to size, let's say a 8.5" width rim. How wide are these tires running? And from a 255 to 285 to 315, what is the difference?

I would guess once you get past the rim width the tires increase would diminish?

I like the width of 285s and would like to get up to 33-34" actual size so assume I would need a 315, but does that actually add 1.25" to width?

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Old 03-14-2016, 06:08 PM   #2
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The tire websites and manufacturers will list a measured rim width and the section width for a given tire. For example a BFG MT KM2 in 35X12.5X17 is listed as a 12.5 section width measured on a 10" rim. So its largest width is 12.5" on a 10" size rim. The general rule I have seen is that for every 0.5" difference in rim width from the measured rim width you gain or lose 0.20-0.25" in section width. So if that same tire was mounted on a 8.5" rim the section width would be something like 11.75-11.9".

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Old 03-14-2016, 08:03 PM
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Yeah, but they also say that their tires are much taller then the actually are. My 285/70/17 are truly 11.5" wide with a tread width of 9.5" on a 8.5" rim. So how wide actually are 315s?
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Old 03-15-2016, 12:22 AM   #4
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A tire measurement is with the tire mounted on a rim with inflation for maximum load, but no actual load on the tire. Yes they will be shorter than advertised under load. The tire manufacturer has no idea how you are going to load the tire so there is no consistent way to give a loaded tire measurement.
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Old 03-15-2016, 12:34 AM   #5
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I swear by this tire calculator.
Click on "tire size calculator" first.

But all tire sizes are "nominal" numbers, not actual measure...
sort of like lumber... a 2x4 is not really a 2x4.

https://tiresize.com/calculator/
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Old 03-15-2016, 09:59 AM
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So from what answers I have gotten it looks like manufacturers are able to get the width of the tire spot on but cannot get the diameter within an inch? Seems strange, but ok.
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Old 03-15-2016, 10:24 AM   #7
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Originally Posted by stumblinhorse View Post
So from what answers I have gotten it looks like manufacturers are able to get the width of the tire spot on but cannot get the diameter within an inch? Seems strange, but ok.
Tire sizes are done by math, they can vary quite a bit in size. Diameter is slightly different because of course a tire will be shorter once there is weight on it, but even widths can vary by manufacturer quite a bit.

For example a 315/70R17;

315 x .7 = 220.5 (sidewall height)
220.5/25.4 = 8.68 (converting mm to inches)
8.68 x 2 = 17.36 (two sidewalls)
17.36 + 17 = 34.36" diameter

And 315 mm width is 12.4" width, most tire companies will list very close to those numbers in their specs but will also put a disclaimer in there about sizing differing slightly.

Anyway to answer the question, as you put a tire on a narrower wheel it crowns the tire and makes the middle of the tread push up a bit and the sidewalls, edges tuck inward. Really though unless you're talking like 3" difference in wheel widths, or running way too narrow of a wheel or something, it's not a huge difference in the real world when you are talking about larger truck sizes since there is a lot of sidewall to soak up the width difference. With a small 40 series car tire or something, it's more important.
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Old 03-15-2016, 12:23 PM   #8
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You have to remember that tire sizes, particularly diameter, are marketing numbers, not measurements or calculations. A "35" is a 35 not because it measures 35", but because it's the next size up from that manufacturer's "33". Some are a little larger than others, some are a little smaller, but it's rare for a tire to measure at its claimed diameter.

Of course, tire measurements are taken unweighted. Anyone who thinks a tire should measure at its advertised diameter on the vehicle is either naive, or should have studied harder in geometry class.

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Old 03-15-2016, 12:59 PM   #9
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The claim is every 1/2" difference in wheel width, equates to a 1/4" difference in diameter.
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Old 03-15-2016, 02:37 PM   #10
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The claim is every 1/2" difference in wheel width, equates to a 1/4" difference in diameter.
1/2" in tire height, diameter equals 1/4" in ride height, lift
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Old 03-15-2016, 04:30 PM   #11
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1/2" in tire height, diameter equals 1/4" in ride height, lift
Yep, that is fact.

I said wheel width though. A wider or narrower wheel (from the baseline test wheel used by the manufacturer, to get a specific height for any given manufacturers advertised tire size. Not what's on the sidewall but advertised diameter) effects the overall tire diameter by changing the width of the sidewall. A wider wheel will pull the overall height down and a narrower wheel will push the height up.
I just read it on a manufacturers website when I was researching tire sizes. I can't recall which manufacturer it was but they stated that a 1/2" change in wheel width, will effect the diameter by up to a 1/4" up or down, depending on if the wheel is wider or narrower than the measured test width wheel that was used to get the number.
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Old 03-15-2016, 05:24 PM
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So that is good news. I am looking to narrower and taller 315/70/17. So put them on 8" rims...
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Old 03-15-2016, 05:53 PM   #13
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Yep, that is fact.

I said wheel width though. A wider or narrower wheel (from the baseline test wheel used by the manufacturer, to get a specific height for any given manufacturers advertised tire size. Not what's on the sidewall but advertised diameter) effects the overall tire diameter by changing the width of the sidewall. A wider wheel will pull the overall height down and a narrower wheel will push the height up.
I just read it on a manufacturers website when I was researching tire sizes. I can't recall which manufacturer it was but they stated that a 1/2" change in wheel width, will effect the diameter by up to a 1/4" up or down, depending on if the wheel is wider or narrower than the measured test width wheel that was used to get the number.
NO !

If the tire has a 10" tread.... a 10" rim will give the tallest height....
narrower or wider will adjust that height but only by minute amounts.
Put that tire on a 6" rim???? 4" narrower and use your "formula" would mean the tire would be 2" shorter.....
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Old 03-15-2016, 08:57 PM   #14
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NO !

If the tire has a 10" tread.... a 10" rim will give the tallest height....
narrower or wider will adjust that height but only by minute amounts.
Put that tire on a 6" rim???? 4" narrower and use your "formula" would mean the tire would be 2" shorter.....

It was section width vs. rim width that I was thinking of, not height. I was mistaken in what I recalled reading and had the two mixed up.

This is what I was reading for reference...
Tire Tech Information - Tire Specs Explained

Ooops.
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Old 03-15-2016, 09:12 PM   #15
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whatever.... hardly counts in the real world.
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Old 03-15-2016, 09:18 PM   #16
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whatever.... hardly counts in the real world.
OK fella

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