Understanding Tire Sizes & Ratings
Understanding Tire Sizes and Ratings: What the Numbers on Your Tires Mean
Two tires can fit the same wheel and still be very different tires. Load capacity, speed rating, construction, sidewall strength, tread compound, approved wheel width, inflation requirements, and intended use can all affect how a tire performs. Understanding those specifications helps you choose a tire that actually matches your vehicle instead of simply buying one that happens to fit.
This article explains tire sizes, load ratings, speed ratings, construction, UTQG markings, inflation information, DOT dates, and other specifications printed on or associated with a tire.
A Tire Is More Than Its Diameter
When customers compare tires, it is easy to focus on the wheel diameter or price. But two tires designed for the same 20-inch wheel can have very different load capacities, sidewall construction, speed ratings, tread compounds, tread-life warranties, inflation limits, ride quality, road-noise characteristics, and intended uses.
That means a tire can physically mount to the wheel and still be a poor choice for the vehicle.
Safety & Braking
Tire construction, tread design, load capacity, inflation, and condition all influence how the tire maintains contact with the road.
Comfort & Noise
Sidewall stiffness, tire weight, tread design, and construction can make two tires of the same nominal size feel completely different.
Load & Towing
Trucks, work vehicles, and towing applications require careful attention to load index, load range, pressure, axle loading, and the vehicle manufacturer's limits.
The Most Important Rule
A tire physically fitting the wheel does not automatically mean it is appropriate for the vehicle. Tire selection should begin with the vehicle manufacturer's requirements and then consider how the vehicle is actually used.
How to Read a Tire Size
A common tire marking may look like this:
Section Width
The tire's nominal section width in millimeters. It is approximately the width from sidewall to sidewall under standardized measuring conditions.
Aspect Ratio
The sidewall height is 55% of the tire's section width. This is a ratio, not a measurement in millimeters.
Radial Construction
The R identifies radial construction. Nearly all modern passenger vehicles use radial tires.
Wheel Diameter
The tire is designed for a 20-inch wheel. Wheel diameter must match exactly. A 20-inch tire cannot be installed on an 18- or 22-inch wheel.
Lower-Profile Tires
- Shorter sidewall
- Generally quicker steering response
- Less sidewall flex
- Potentially firmer ride
- Less sidewall cushioning over potholes and impacts
Higher-Profile Tires
- More sidewall cushioning
- Greater impact absorption
- More protection from pothole impacts
- More sidewall flex
- Potentially softer ride characteristics
Changing One Number Changes the Tire
Changing width, aspect ratio, or wheel diameter can change the tire's overall dimensions. On modified vehicles, tire sizing should be calculated rather than guessed because overall diameter can affect clearance and vehicle systems.
What Is Tire Load Index?
In the example 275/55R20 116S, the number 116 is the tire's load index.
Load index is a numerical code corresponding to the maximum load the tire is designed to carry under specified operating conditions.
| Load Index | Approx. Maximum Load Per Tire | What It Means |
|---|---|---|
| 116 | 2,756 lb | Common in some passenger, SUV, and light-duty truck applications. |
| 121 | 3,197 lb | Provides greater rated carrying capacity under specified conditions. |
| 125 | 3,638 lb | Often found in heavier-duty tire applications. |
A Higher Tire Load Index Does Not Increase Vehicle Payload
Installing tires with a higher load capacity does not increase the vehicle manufacturer's GVWR, GAWR, payload rating, towing capacity, axle capacity, wheel rating, or suspension capacity.
The vehicle is still limited by the ratings assigned by the vehicle manufacturer and by its lowest-rated applicable component.
What Does a Dual Load Rating Mean?
Some LT and commercial tires may show a rating such as:
125 Load Index
Approximately 3,638 pounds per tire under the specified conditions for single-wheel use.
122 Load Index
Approximately 3,307 pounds per tire when used as part of a dual-wheel configuration.
The individual dual-tire rating is typically lower because dual-wheel installations have different operating conditions. The tire's published single and dual ratings should be used for the configuration in which it is actually installed.
What Does the Tire Speed Rating Mean?
The letter after the load index is the speed rating. In 116S, the letter S is the speed rating.
| Rating | Approx. Rated Speed |
|---|---|
| Q | 99 mph |
| R | 106 mph |
| S | 112 mph |
| T | 118 mph |
| H | 130 mph |
| V | 149 mph |
| W | 168 mph |
| Y | 186 mph |
A Speed Rating Is Not Permission to Drive at That Speed
The rating represents the tire's ability to withstand specified heat and forces during standardized testing. It also commonly reflects differences in construction, heat resistance, handling characteristics, and sidewall behavior.
The tire should generally meet or exceed the vehicle manufacturer's required speed rating unless there is a legitimate application-specific reason to use a different specification.
What Does 4-Ply, 10-Ply, or 12-Ply Rated Mean?
Modern ply ratings do not necessarily mean the tire literally contains that number of fabric layers. Historically, additional plies were used to describe stronger tire construction. Today, a ply rating is primarily a strength and load-capability classification.
4-Ply Rated
Generally associated with lighter-duty passenger applications.
- Lower tire weight
- Softer ride characteristics
- Lower rolling resistance in many applications
- Good everyday comfort
10-Ply Rated / Load Range E
Common on heavy pickups, work trucks, towing vehicles, and some commercial applications.
- Higher load capability
- Stronger casing
- Less deformation under heavy load
- Higher allowable inflation pressures
12-Ply Rated / Load Range F
Generally intended for still heavier tire applications where additional load capability is required.
- Heavy-duty construction
- Higher load capability in applicable sizes
- Often higher tire weight
- Typically firmer ride characteristics
Higher Ply Rating Is Not Automatically Better
A heavy commercial tire may provide no useful advantage on a lightly-loaded daily driver. More tire weight and stiffer construction can produce a firmer ride and may affect fuel economy and suspension response.
The correct tire is the tire that matches the vehicle and its actual use.
P-Metric Tires vs. LT Tires
Passenger tires and light-truck tires can sometimes be available in similar physical sizes, but changing between them affects much more than appearance.
P-Metric / Passenger Tires
- Designed primarily for passenger-vehicle use
- Typically lighter construction
- Often lower tire weight
- Generally more comfort-oriented
- Usually lower required inflation pressures than heavy LT applications
LT Tires
- Designed for light-truck and heavier-duty applications
- Stronger construction
- Higher load capability in applicable sizes
- Higher inflation capability
- Common for towing, hauling, work, and commercial use
Installing an LT tire in place of a passenger tire can change sidewall stiffness, tire weight, required inflation strategy, ride quality, and handling. The change should be evaluated as a complete application decision instead of assuming that a heavier tire is automatically an upgrade.
What Does The UTQG Rating Mean?
Many passenger tires carry a Uniform Tire Quality Grading, or UTQG, marking. An example may look like:
Treadwear
A relative treadwear rating. A higher number generally indicates a longer-wearing compound, but the number should not be interpreted as a direct mileage guarantee.
Comparisons are most useful among similar products and should be considered along with the manufacturer's actual treadwear warranty.
Traction
UTQG traction grades include: AA, A, B, and C.
The test primarily evaluates wet straight-line braking under specified conditions. It does not completely describe dry grip, cornering, snow traction, or hydroplaning resistance.
Temperature
Temperature grades include A, B, and C.
This portion of the grading system relates to resistance to heat buildup and the tire's ability to dissipate heat under the specified test conditions.
Why Does My Tire Show No UTQG?
Many LT, commercial, temporary-spare, and other specialized tires are not required to carry the same UTQG markings found on typical passenger tires.
A blank UTQG field or missing UTQG marking does not automatically mean the tire is low quality.
What Does a 55,000- or 65,000-Mile Tire Warranty Mean?
A mileage figure associated with a tire usually refers to the manufacturer's limited treadwear warranty.
A 65,000-mile warranty does not mean every customer is guaranteed exactly 65,000 miles of usable tread.
Warranty claims may also require evidence that the tires were properly maintained and rotated. The warranty terms should be reviewed before assuming a mileage figure is a guarantee of actual tire life.
What Does Approved Rim Width Mean?
A tire specification may list an approved wheel-width range such as:
A tire is engineered to operate within a specific range of wheel widths. Installing it on a wheel that is too narrow or too wide can change the tire's shape and how it operates.
“It Fits” Is Not the Same as “It Is Approved”
A tire may appear to mount onto a wheel even when that wheel width is outside the tire manufacturer's approved range. Tire and wheel specifications should be verified before installation.
Maximum Tire Pressure vs. Recommended Vehicle Pressure
The pressure molded into the tire sidewall and the pressure listed on the vehicle's tire-information placard are not the same thing.
Maximum Tire Pressure
The tire sidewall identifies maximum-load and inflation information associated with the tire's design and rating.
That maximum value is not automatically the correct everyday inflation pressure for the vehicle.
Recommended Vehicle Pressure
For the original approved tire application, normal inflation should generally be based on the pressure specified by the vehicle manufacturer on the tire-information placard.
This placard is commonly found in the driver's door-jamb area.
Inflating a tire blindly to the maximum pressure printed on its sidewall can negatively affect ride comfort, tire wear, traction, and handling.
Modified, Towing, and Commercial Applications Need More Consideration
Vehicles using non-original tire sizes, LT conversions, commercial applications, or unusual loading may require additional evaluation. Pressure should be based on the actual tire, vehicle, load, and approved application rather than guessing.
How to Read a DOT Tire Date Code
A tire's DOT identification includes a manufacturing date code. The last four digits represent the week and year the tire was manufactured.
Tire age matters even when plenty of tread remains because rubber and tire components can change over time.
Tread Depth Is Not the Only Indicator of Tire Condition
Tire condition, manufacturer guidance, cracking, damage, service history, vehicle requirements, and professional inspection should all be considered. There is not one universal age rule that applies identically to every tire and every vehicle.
Other Specifications You May See
BLK / BSW
These commonly indicate a black sidewall.
OWL / RWL
These refer to outlined or raised white-letter sidewall designs.
XL / Extra Load
Extra Load identifies a reinforced passenger tire designed to carry greater loads at higher inflation pressures than a comparable standard-load passenger tire.
XL is not the same thing as an LT tire.
M+S
M+S means Mud and Snow and is based primarily on specified tread design characteristics.
3PMSF
The Three-Peak Mountain Snowflake symbol indicates the tire has met a defined snow-traction performance requirement.
M+S vs. 3PMSF
These markings should not be treated as identical certifications. A tire carrying the 3PMSF symbol has met a defined snow-performance test requirement beyond the tread-design basis associated with M+S.
How to Compare Two Tires That Look the Same
Imagine two tires are both advertised in the same basic size. Before assuming they are interchangeable, compare the complete specification.
| Specification | Why It Matters |
|---|---|
| Tire Size | Determines width, sidewall ratio, construction, and wheel diameter. |
| Load Index | Determines the tire's rated load capability under specified conditions. |
| Speed Rating | Reflects standardized heat and high-speed capability. |
| Load Range | Relates to tire strength, inflation capability, and heavy-load use. |
| Passenger vs. LT | Can substantially change weight, stiffness, inflation, ride, and load characteristics. |
| UTQG | Provides relative treadwear, traction, and temperature grades where applicable. |
| Approved Rim Width | Ensures the tire is operating on a wheel width it was designed to use. |
| Treadwear Warranty | Provides manufacturer warranty information but does not guarantee exact tire life. |
| DOT Date | Shows when the tire was manufactured and helps evaluate age. |
Specifications That Are Often Misunderstood
Myth: More Ply Rating Always Means a Better Tire
Fact: Higher load capability can be useful, but a heavier and stiffer tire can reduce comfort and efficiency when the vehicle does not need that additional capacity.
Myth: If It Fits the Wheel, It Is Safe
Fact: Wheel diameter is only one requirement. Load rating, width, overall dimensions, approved rim width, speed rating, and vehicle specifications also matter.
Myth: Inflate It to the Number on the Tire
Fact: Maximum tire pressure is not automatically the correct normal operating pressure. Factory-size applications generally use the vehicle manufacturer's placard pressure.
Myth: The Highest Load Index Is Always Best
Fact: The correct load index is the one that properly supports the vehicle and application while meeting the manufacturer's requirements.
What Should Be Verified?
The Best Tire Is Not Necessarily the Most Expensive Tire
It is also not automatically the tire with the highest load index or stiffest construction.
The best tire is the tire that properly matches the vehicle, wheel, manufacturer's requirements, and the way the vehicle is actually used.
Not Sure What Tires Your Vehicle Actually Needs?
Portillo Automotive can help inspect your tire size, load and speed ratings, inflation, tread condition, wear patterns, wheel compatibility, and suspension-related tire wear so you can understand what is currently installed and what your vehicle actually requires.
Tire requirements vary by vehicle, wheel, load, modification, tire manufacturer, and intended use. Always verify the vehicle manufacturer's requirements and the tire manufacturer's specifications before changing tire size, type, load rating, or inflation strategy.