From Billa vista "Tire Sizing Systems
There are many tire sizing systems still in use today, some dating back to the 1930’s and others having come into use only since the 70’s and 80’s. I will explain each one, as you are likely to see any and all of these systems when shopping for off-road tires (Interco themselves use four different systems: Alpha-Numeric, Metric, Flotation, and LT-Numeric).
Numeric
Earliest system, dates back to the 30’s when there were few tire sizes available (or needed)
Example:
9.15-15
9.15 is the section width in inches
15 is the rim diameter in inches
Notes:
- Using this system you can’t tell the OD of the tire without knowing the Aspect Ratio
- In the 1950s and early 1960s, the standard aspect ratio was 82-84%
- Around 1970, bias ply tires moved to a 78% aspect ratio
- In the 70’s the "standard" aspect ratio dropped to 75%
- Today, Aspect Ratios of 65%, 60% and even as low as 35% are available.
- If you know the AR of the tire in question you can calculate the OD as: (Section Width x Aspect Ratio) x 2 + Rim Diameter (eg. in the above example, if we knew the Aspect Ratio were 78%: (9.15 x 0.78) x 2 + 15 = 29.3" )
- If you don’t know the AR, the only way to tell the OD is to consult the manufacturer’s specifications.
Alpha Numeric
This system began to appear in the late 1960’s. It adds the Load Rating and Aspect Ratio information, but mysteriously drops the Section Width.
Example:
H78-15
H – load rating
78 – Aspect ratio in %
15 – diameter in inches
Notes:
– This is an almost useless system as we don’t know, nor can we determine, the section width or OD of the tire.
– When radial tires began to appear, an R was added to the designation but this still doesn't’t help us determine size:
Example:
HR78-15
– There are some charts available that list common Alpha-Numeric sizes and the equivalent OD and Section Width of the tire, in inches, but these must be used with caution as they are by no means universal. Here is one such chart:
L78/15 = 30x 9.5
N78/15 = 31x 9.5
P78/15 = 33x10.0
Q78/15 = 36x11.5
Q78/16 = 36x10.5
R78/15 = 37x12.5
R85/16 = 37x12.0
-Though the alpha-numeric system doesn’t tell us the OD, we can, however, tell the relative size of tires using this system. Here’s why:
There are only two ways to increases the load carrying capacity of a tire
1) Increase the max pressure it can hold (by altering its construction/composition) or
2) Increase the size of the tire so there are more Square Inches for each Pound of air pressure to push against (a tires load in Pounds is roughly it’s max pressure (PSI) multiplied by the number of Square Inches of its contact patch)
E.g. Load = 50 P/SI x 30 SI = 1500 lbs
The alphabetic load rating system was originally based on this principle, with higher letters being assigned to larger tires able to carry more load. For this reason, we can also say that, for tires using the alpha-numeric system, the higher the letter, the larger the tire. For example, we can see from the preceding chart that an N78/15 (31x9.5) is smaller than a Q78/15 (36x11.5)
- Again, the best way to determine the dimensions of an Alpha-Numeric tire is to consult the manufacturer’s specifications.
Metric (Euro-Metric, P-Metric)
The Metric system first appeared in the 1970’s and was called the Euro-Metric system because it began in Europe. It is the first system that tells us virtually all we need to know, though some information must be calculated from the figures given.
Example:
235/75R15
235 is the section width in mm
75 is the Aspect Ratio in %
R denotes radial construction (D = (diagonal) bias ply construction; B = belted bias construction)
15 is the rim diameter in inches
To calculate the complete dimensions:
First change section width to inches by dividing the mm by 25.4
235/25.4 = 9.25” wide
Then calculate the OD in inches by multiplying the Section Width by the Aspect Ratio (to get the height of the sidewall), doubling it, and adding the diameter of the rim
OD = ((9.25*.75)*2)+15 = 28.87 ~ 29”
Therefore a 235/75/R15 is a 29” x 9.25” R15
Notes:
- This system has becom widely known as the “P-Metric” system as a P appears at the beginning of modern designations to indicate a “Passenger car” tire. Although the system is also used for light truck and special trailer tires.
Examples:
P235/75R15
LT 275/70R16
ST 205/75R14
- System allows rapid comparison of the AR or “profile” of the tire
- Unlikely to ever see anything but an R in P and LT tires, but may see a D or B for certain special tires like ST tires
- The D.O.T. decided to add the "P" to P-Metric to denote that it was designed for passenger car use for load carrying capacity limits.
- Note that P and LT tires are often rated slightly different. If P-rated tire is used in an LT application, you need to de-rate the sidewall load capacity by 9% - 10% according to many tire makers.
- The best system so far, but not perfect (for us) as it requires calculations to give the complete dimensions which we need (because we run such a VAST range of different sizes)
- Early metric sizes may be shown without the AR number
Example:
165SR15
This is because the metric system was implemented in Europe when tires were still chiefly bias ply, available only in a limited number of sizes (due to construction technology of the time) and all using a standard aspect ratio of "82". Therefore, originally it was not necessary to include the AR in the sizing system. With the advent of radial tire construction, tire manufacturer’s were suddenly able to construct better performing street tires by increasing the section width and reducing the sidewall height of the tire. In doing so, they began making tires with aspect rations lower than the “standard” 82% and so it became necessary to include the AR in the metric tire sizing system
Flotation
The Flotation Sizing system is used for larger, wider tires (LT tires) used on trucks and SUV’s – so named because these tires are supposed to “float” over soft surfaces. It is a convenient system for us as it gives the whole picture at a glance.
Examples:
39.5x13.50-16LT
36x12.50R15
36 = OD in inches
12.50 = Section width (NOT tread width)
R = denotes radial construction (absence of the R denotes bias ply construction)
15 = rim diameter in inches
LT = Light Truck tire (may or may not be present)
Tread width may be published by manufacturer, but for us will vary enormously anyway with load and low air pressures.
LT Numeric
For LT tires there also exists another useful system – the LT numeric system – based on the old Numeric system but with the useful addition of the OD.
Example:
9/34-16LT
9 is the approximate section width (in inches)
34 is the approximate overall tire diameter (in inches)
16 is the rim diameter in inches)
LT designates a Light Truck application
Notes:
- This system is used only for bias ply tires.
Summary:
Numeric – old and practically useless (unless you’re just shopping for an exact replacement tire from the same manufacturer for your tractor, small trailer, or wheelbarrow). Tires in these sizes will be bias ply.
Alpha-Numeric – only slightly less useless than the numeric system. Might be of some use to those competing in certain sanctioning bodies with rules based on what is written on the tire. E.g. getting away with an aired-down Q-78/15 in a 35” and under class. May be used for radial and bias ply tires.
Metric – has all we need to know, but calculations must be made. Not common in the sizes hardcore wheelers need (38” OD and up). Used with radial and bias ply tires, indicated by an R,D, or B in the designation. Older metric sizing on bias ply tires used a standard, unstated AR of 82%.
Flotation – Most useful “at a glance” info for us, used with both radial and bias ply tires where a radial will always have an R in the designation but a bias may just have no letter. May or may not include the designation “LT” at the end.
LT-Numeric – Similarly useful as the Flotation system, if a little more complicated to read. Used only for LT tires, and only for bias ply construction.
Both the Flotation and LT-Numeric sizing systems tell us the basics of what we need to know. My preference is for the Flotation system as it tells me the info in an easy to read format in the order that I most care about – OD, then width, then rim diameter. Then again, it might just be because that’s the system I grew up reading (and often dreaming) about!"
More on the subject
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