Short definitions for tire terms that show up on sidewalls, work orders, inspection notes, and fleet policies.
A letter-based strength classification for tires that groups them by maximum cold inflation pressure and load capacity. Common highway truck tire load ranges are G (maximum 110 PSI) and H (maximum 120 PSI), with F (95 PSI) in some lighter applications and L (130 PSI) for heavy vocational use. The load range letter appears on the tire sidewall as part of the service description — for example, "G" or "LR G" — and corresponds to a specific maximum cold inflation pressure and maximum rated load at that pressure. The correct load range for an application is determined by comparing the actual axle load to the manufacturer's load and inflation table for the specific tire model and size.
A numeric code appearing in a tire's service description that corresponds to a specific maximum load capacity in a published table. Commercial truck tires typically use a two- or three-digit load index — for example, "149/146" on a dual-rated tire, where the first number is the single-service index and the second is the dual-service index. The actual load capacity in pounds for each index number is found in the published load index table. For commercial truck applications, the load range designation often provides more direct decision-making information, but the load index is part of the complete service description used to verify the tire is correct for the application.
A historical shorthand for tire strength that originated in the era of bias-ply tires, where the number of actual cord plies determined strength. Modern radial truck tires are not built with the number of cord layers the ply rating suggests — a "14-ply" radial tire may have only two or three structural cord plies, engineered to equivalent or greater strength through advanced casing design. In current use, ply rating is a shorthand for load range categories: 12-ply corresponds to Load Range F, 14-ply to Load Range G, 16-ply to Load Range H, and 20-ply to Load Range L. The load range letter is the more precise and current identifier.
The part of the sidewall marking that states load index, speed symbol, and sometimes single or dual designation — separate from the size string.
A tire manufacturer's published chart showing the maximum load capacity for a specific tire at each inflation pressure. The required reference for setting pressure to match actual axle load.
Gross Axle Weight Rating — the maximum rated weight for a single axle system as set by the vehicle manufacturer, accounting for the combined capacity of the suspension components, axle housing, wheel bearings, wheels, and structural connections to the frame. GAWR appears on the vehicle certification label for each axle position separately — front and rear axles typically have different ratings. The GAWR is a manufacturer-certified limit and cannot be raised by changing tires alone. Both GAWR and tire load capacity apply simultaneously; the lower of the two controls the maximum permissible axle load.
Gross Vehicle Weight Rating — the maximum rated weight of the entire vehicle as certified by the manufacturer, covering the chassis, frame, axles, body, payload, driver, and fuel. For commercial vehicles, the GVWR appears on the vehicle certification label alongside individual GAWR values for each axle. Federal highway regulations for Interstate operation set a gross vehicle weight limit of 80,000 lb for standard configurations, but the vehicle-specific GVWR may be different — it is a manufacturer equipment limit that applies independently of road regulations.
The ratio of the tire's sidewall height to its section width, expressed as a percentage. In 295/75R22.5, the "75" means the sidewall height is 75% of the 295 mm section width.
The distance between the outer sidewalls at the widest point of an inflated, unmounted tire, measured in millimeters on metric-designated tires. In the size 295/75R22.5, "295" is the nominal section width in millimeters. For older alpha-numeric sizes like 11R22.5, the width is expressed differently — "11" refers to the nominal width in inches, approximately 279 mm. Section width determines whether a tire fits within fender clearances, maintains adequate spacing in a dual assembly, and falls within the tire manufacturer's approved rim width range. A tire's actual inflated section width may differ slightly from the nominal width depending on the rim width it is mounted on.
The outer diameter of an inflated, unloaded tire. Determines ride height, fender clearance, gearing effect, speedometer accuracy, and whether two tires can be used together in a dual set.
The interior width of the wheel measured between the flanges. Tire manufacturers specify a range of approved rim widths for each tire size. Using a rim outside the approved range affects tire shape, capacity, and handling.
The distance between the centerlines (or adjacent sidewalls) of two tires mounted in a dual assembly. Minimum dual spacing must be maintained to prevent sidewall contact under load.
A tire mounted on the front steering axle of a tractor or truck — the position that directly controls steering response, handling stability, and braking feel. Unlike drive and trailer tires, steer tires operate in single-tire service with the full steering load transferred through them on every turn. They face forward load from the engine and fifth-wheel weight plus lateral cornering forces. Federal tread depth minimums are stricter for steer axle tires — 4/32 inch versus 2/32 inch at other positions — reflecting the higher safety consequences of steer tire failure.
A tire mounted on a powered drive axle — the axle or axles that receive torque from the driveshaft. On a standard highway tractor, the drive axles are the tandem rear axles, each typically carrying four tires in dual assemblies for a total of eight drive tires. Drive tires transmit traction, braking, and torque forces to the road, which subjects them to different wear patterns than steer or trailer tires. Lug-pattern drive tires with wider grooves are common in applications requiring traction; rib-pattern drive tires are used in fuel-efficiency highway programs.
A tire mounted on a trailer axle — the rear axle or axles on the pulled unit rather than on the tractor. Trailer tires are almost always run in dual assemblies on standard van, reefer, and flatbed configurations, though wide-base singles are used on some fuel-optimized trailers. They handle lower steer forces than tractor front tires but face scrubbing from tight turns, off-center cargo loading, and irregular maintenance intervals — trailer custody changes frequently in pool fleets, so consistent inspection history is rarely guaranteed.
One of two tires mounted side by side on the same wheel end, sharing the load of a drive or trailer axle position. The dual load rating for any tire is lower than its single rating because close proximity limits heat dissipation and requires precise load sharing. For dual assemblies to work correctly, both tires must be matched: same size, compatible overall diameter (typically within 1/4 inch), same or very similar tread depth (typically within 4/32 inch), and the same target inflation pressure. A mismatch in any of these produces unequal load sharing, heat, and accelerated wear.
A tire used alone on a wheel end rather than paired with a second tire as a dual assembly. Steer axle positions on a standard highway tractor are almost always single-tire service. Wide-base single tires also replace dual assemblies on drive and trailer positions in some fuel-optimized configurations. The load rating for a tire in single service is typically higher than the same tire's dual service rating — in single service the tire has better heat dissipation and is not affected by load-sharing inconsistencies from a mismatched dual mate. Always use the single-position rated capacity from the manufacturer's table when the tire is used alone.
A single wide tire that replaces a dual assembly per wheel end, also called a super single. Requires a wider wheel, has a different load rating structure, and changes spare-tire logistics.
A tire with cord plies running radially from bead to bead at 90 degrees to the direction of travel, stabilized by belt plies under the tread. Virtually all modern commercial truck tires are radial, indicated by the "R" in size markings.
Tire pressure measured before the vehicle has been driven more than one mile or after it has sat for at least three hours away from direct sunlight. The standard baseline for setting and comparing pressure.
Tire Pressure Monitoring System — an electronic system that monitors tire pressure in real time and alerts the driver when pressure drops below a defined threshold.
The structural body of the tire beneath the tread rubber — comprising the radial cord plies, steel belt layers, bead wire bundles, and sidewall rubber — that gives the tire its load-carrying capability and shape. The casing is the long-term investment in a commercial tire: premium casings from recognized tire brands are designed to survive multiple retread cycles, extending their value beyond the first tread life. Casing condition at removal determines retreading eligibility — a casing run flat, severely underinflated, improperly repaired, or damaged by a road hazard may be rejected at retreader inspection even if it appears serviceable from the outside.
The depth of usable tread measured in major grooves with a proper gauge. Federal minimums: 4/32 inch on steer axles, 2/32 inch on other positions (49 CFR 393.75).
A raised bar molded into major tread grooves at approximately 2/32 inch depth. When the tread surface wears flush with the bar, the tire has reached the federal minimum for non-steer positions.
The flexible section of the tire between the tread shoulder and the bead — the area that compresses and rebounds under load with each revolution. The sidewall carries the tire's key markings: size string, service description, load range, maximum cold inflation pressure, TIN (Tire Identification Number), and the "RETREAD" or "RECAPPED" marking if applicable. Unlike the tread, which is a wear surface, the sidewall is not designed for road contact — any condition that removes rubber from the sidewall or exposes the internal cord structure is a serious defect requiring immediate action.
The last four digits of the Tire Identification Number — first two digits are the production week (01–52), last two are the year. Example: "2524" = week 25 of 2024.
The full identifier molded into the tire sidewall starting with "DOT" — includes manufacturer code, plant code, tire type/size code, and the four-digit date code. Required by 49 CFR Part 574.
The device through which a tubeless tire is inflated and pressure is checked. Valve stem failure — from deterioration, damage, or a leaking core — is a common source of slow pressure loss.
An enforcement action requiring a commercial vehicle or driver to stop operating until a defect is corrected. Tire conditions that commonly trigger out-of-service orders include tread below minimums, visible cord, flat tires, and sidewall bulges.
A sudden lateral sidewall separation that occurs when a tubeless tire run severely underinflated or flat is reinflated. The internal structure may have separated without visible exterior signs, then releases rapidly when pressure is applied.
A tire produced by inspecting a used casing, buffing its surface to a precise profile, optionally repairing damage that meets retreader standards, and bonding new tread rubber through a controlled vulcanization process. There are two main methods: pre-cure, where a pre-molded tread band is applied and vulcanized, and mold cure, where uncured tread rubber is shaped in a heated mold around the casing. Quality depends on the casing's condition and history, the retreader's inspection standards, the tread compound quality, and the precision of the curing process. A quality retread on a suitable casing, in an appropriate position, can deliver comparable performance to a new tire in many drive and trailer applications.
Common trucking slang for a retread tire — both terms refer to the same product: a used casing that has been inspected, buffed, and retreaded with new tread rubber through a controlled vulcanization process. The term "recap" is widely used at truck stops, in driver conversations, and in maintenance records, but it has no different technical meaning from "retread." Federal regulations use the terms "retread" and "recapped" in tire marking requirements. The performance, inspection requirements, and position restrictions for a recap are exactly the same as for any retread tire.
A monetary credit a retreader or tire supplier applies toward a purchase in exchange for a returned, inspected casing. The credit amount depends on casing condition and whether it passes the retreader's acceptance inspection.
A failure mode where tire layers — typically tread-to-belt or belt-to-belt — come apart due to heat damage from underinflation, overloading, or high-speed operation.
The complete assembly at each wheel position including hub, bearings, seals, spindle, and mounting hardware. Wheel-end problems such as worn bearings can cause cupping wear that looks like a tire issue.
An alignment angle describing how the front edges of a pair of tires point relative to the direction of travel. Toe-in points inward; toe-out points outward. Incorrect toe is the most common cause of feathering wear on truck tires.
The adjustment of wheel angles — including toe, camber, and caster — to manufacturer specifications. Correct alignment is essential for even tire wear, predictable handling, and fuel efficiency.
The practice of moving tires between axle positions to equalize wear patterns. Common drive-axle rotation moves front drive to rear drive and vice versa. Cross-rotation between steer and drive positions requires confirming tire approval for both positions.
The force required to keep a tire rolling at constant speed. Lower rolling resistance reduces fuel consumption. Tire design, inflation, load, temperature, and speed all affect rolling resistance.
A code indicating the maximum speed at which a tire is designed to carry its rated load over an extended period. Commercial truck speed symbols: L = 75 mph, M = 81 mph, N = 87 mph. Appears as part of the service description.
The proportion of the tread face that is open groove space versus rubber. Higher void ratios improve wet traction and drainage; lower void ratios provide more rubber contact for wear resistance and fuel efficiency.
The reinforced edge of a tire that seals against the wheel rim flange and holds the tire in position. The bead is constructed of high-tensile steel wire bundled in rubber. Bead damage — from curb impact, mounting errors, or running flat — can compromise the seal and may not be repairable.
Reinforcing layers of steel or other cord material located between the tread rubber and the radial casing plies. Belts stabilize the tread, control tread-face shape under load, and resist penetration. Belt separation — where a belt layer comes apart from adjacent layers — is a serious structural failure.
The innermost layer of a tubeless tire — a specialized rubber compound that functions as the air barrier, replacing the inner tube used in older bias-ply tires. Damage to the innerliner from running flat or from improper repair techniques can cause slow air loss even without a visible external defect.
The transition zone where the tread face meets the tire sidewall. The shoulder is a high-stress area subject to heat buildup, impact damage, and wear from alignment or underinflation. Shoulder wear — where the outermost tread ribs are lower than the center ribs — often indicates pressure or load problems.
A continuous circumferential ridge of rubber on the tread face, defined by the grooves on each side. Rib tires — used primarily in steer and highway drive positions — have wide, continuous ribs that minimize tread movement and maximize even wear and fuel efficiency.
The open channel between tread ribs or lug blocks that channels water from the contact patch and contributes to wet-weather traction. Federal tread depth measurements are taken in major tread grooves. As grooves wear shallower, wet-weather drainage and traction capability decrease.
A narrow cut (typically less than 1.5 mm wide) made into a tread block or rib to add biting edges for wet, snowy, or icy traction. Sipes are common on steer and all-weather tires. As tread wears, sipes shorten and their traction contribution decreases before the overall tread depth reaches the removal threshold.
The reinforcing fiber material — typically steel wire, nylon, polyester, or aramid — used to construct the structural layers of a tire. Exposed cord in the tread or sidewall is a serious defect and an out-of-service condition. The "cord" is what provides structural strength in the casing; once it is exposed, the tire's structural integrity is compromised.
The metal wheel assembly on which the tire is mounted. For commercial trucks, disc wheels (most common) and spoke wheels are used. Wheel condition — cracks, corrosion at the bead seat, damaged lug holes, and distorted flanges — directly affects tire seating and performance. A damaged wheel can cause persistent bead leaks even with a serviceable tire.
The precision-machined surface on the wheel rim where the tire bead rests and seals. Corrosion, physical damage, or improper mounting technique at the bead seat causes bead leaks that appear as slow pressure loss at the tire-to-wheel interface. The bead seat surface must be clean and undamaged for a proper seal.
The deviation from perfect roundness in a mounted tire and wheel assembly. Lateral runout causes the wheel to wobble from side to side; radial runout causes it to hop up and down. Either type causes vibration at highway speed. Runout can originate from the tire (out-of-round), the wheel (bent or warped), or the mounting (off-center seating).
The even distribution of weight around the mounted tire and wheel assembly. An out-of-balance assembly causes vibration at specific speed ranges, which can accelerate cupping wear and cause driver fatigue. Balance weights are added to the rim to compensate for weight variations in the tire-wheel assembly.
A safety restraint device required by OSHA (29 CFR 1910.177) when inflating tires that may have been run flat or that have unknown prior condition. The cage contains the tire and rim if a zipper failure occurs during reinflation. Using an inflation cage is mandatory in shop settings when the tire's condition is uncertain.
The actual measured load carried by a single axle at any given time. Axle weight varies by cargo loading, cargo distribution, fuel level, and trailer type. The correct tire inflation pressure for each axle position is determined by the actual axle weight, not by a default fleet setting.
Two drive axles mounted close together on a tractor, sharing the drive load. Tandem axles on Interstates are federally limited to 34,000 lb combined weight under 23 U.S.C. 127. Tire rotation on tandem axles typically involves moving tires from the forward drive position to the rear, or swapping inner and outer positions.
The front axle that steers the vehicle. The steer axle carries the forward vehicle weight and all steering forces. Tires on the steer axle face unique demands: full steering side loads, braking forces, and the highest safety implications of any tire position. Federal tread depth minimums are stricter for steer axle tires (4/32 inch) than for other positions.
A federal formula under 23 U.S.C. 127 that limits the weight carried by groups of axles on Interstates and other federal-aid highways, based on axle spacing. The formula prevents concentrated loads from damaging bridge structures. Single axle limit: 20,000 lb. Tandem axle limit: 34,000 lb. Gross vehicle weight limit: 80,000 lb (standard). Exceeding these limits is a violation even if individual tire and axle ratings are not exceeded.
The maximum cold inflation pressure at which a tire can be operated, molded on the tire sidewall. This is a physical limit — not the recommended pressure for all loads. The correct operating pressure, which may be lower, is determined by the actual axle load and the manufacturer's load and inflation table.
A gradual loss of tire pressure that occurs over days or weeks rather than hours. Slow leaks are often undetected between inspections, allowing tires to run progressively lower until a pressure check or failure reveals the problem. Common causes include deteriorated valve cores, porous bead seals, micro-punctures, and innerliner damage.
The small spring-loaded pin inside the valve stem that controls airflow. The valve core seals when not in use and opens when pressed by a gauge, inflator, or cap. A deteriorated valve core is one of the most common causes of slow pressure loss in truck tires. Valve cores are inexpensive and replaceable without removing the tire.
The increase in tire pressure that occurs as the tire heats up during operation. A commercial truck tire typically shows 10–20 PSI above its cold reading after highway operation. This increase is normal and expected. Hot pressure readings should not be used to adjust inflation downward — doing so results in the tire being underinflated when it cools.
A localized area of reduced tread depth on one side of the circumference, caused by wheel lockup during braking, extended parking on a deflated tire, or long-term static storage. Flat spots produce a rhythmic vibration or thump at highway speed. Mild flat spots may partially drive out; severe or structural flat spots require tire removal.
A wear pattern characterized by repeating scalloped dips or high-and-low spots around the tire circumference, usually caused by suspension bounce — worn shock absorbers, loose wheel bearings, or damaged suspension components. Also called scalloping. The irregular contact pattern wears the tread unevenly at the bounce frequency.
Any tread wear pattern that does not progress evenly across the full tread width and circumference. Irregular wear patterns include center wear, edge wear, one-side wear, cupping, feathering, heel-toe wear, and diagonal patterns. Each pattern indicates a specific or combination of causes involving pressure, load, alignment, or suspension.
A condition where the structural fiber material beneath the tread rubber or inside the sidewall has become visible through the outer surface. Cord exposure in the tread (from wear through the tread depth) or in the sidewall (from impact or structural damage) is an out-of-service condition and an immediate removal trigger. A tire with exposed cord should not be operated.
A localized outward protrusion of the tire sidewall caused by internal structural damage — broken cords, separated plies, or belt edge failure at the sidewall. A sidewall bulge indicates that the outer rubber is unsupported by the structure beneath it at that point. The tire is not repairable and must be removed from service immediately.
Surface cracking of the tire sidewall rubber caused by prolonged exposure to ozone, UV light, and weathering. Fine cracks typically appear in the sidewall and shoulder area, running perpendicular to the direction of deflection. Superficial surface cracks may be cosmetic; cracks that open under flexing or penetrate to the cord layer are a structural concern.
A failure mode where the tread rubber separates from the belt package or where belt layers separate from each other, typically beginning at the shoulder and advancing across the tread face. Tread separation can be gradual (giving warning signs) or rapid. It is often associated with underinflation, heat damage, or prior casing fatigue.
The Commercial Vehicle Safety Alliance — an organization of enforcement agencies from the United States, Canada, and Mexico that establishes standardized inspection procedures and out-of-service criteria for commercial vehicles. The CVSA North American Standard Out-of-Service Criteria define the specific thresholds used in roadside inspections to determine when a vehicle or driver should be placed out of service.
Driver Vehicle Inspection Report — a federally required document (49 CFR Part 396) that commercial drivers must complete at the end of each duty day, reporting any defects or deficiencies observed on the vehicle. Tire defects must be noted on the DVIR. The DVIR must be reviewed by the next driver and signed by a qualified mechanic after any required repairs.
The practice of ensuring that two tires mounted in a dual assembly are closely matched in overall diameter, tread depth, and inflation pressure to prevent one tire from dragging or carrying excess load. Industry guidance typically calls for dual mates to be within 4/32 inch of tread depth and within 5 PSI of pressure.
A tool for measuring tread depth in major tread grooves. Probe-type gauges insert directly into grooves and give a direct reading in 32nds of an inch or millimeters. Unlike the coin test (which is not calibrated and not reliable for commercial vehicles), a proper gauge gives a measurement that can be compared to federal thresholds and fleet pull points.
An inspection of a commercial motor vehicle conducted at the roadside by authorized enforcement personnel, typically under DOT authority. Tire defects observed during a roadside inspection can result in out-of-service orders for the vehicle. Common tire-related triggers include tread below federal minimums, visible cord, flat tires, sidewall bulges, and prohibited tire conditions.
A tire application category for high-speed (55 mph or above), primarily Interstate or long-haul operation. Highway-service tires typically use rib tread patterns, low void ratios, and tread compounds optimized for low rolling resistance and long wear life at sustained highway speeds. Fuel economy performance is more relevant in highway service than in mixed or regional service.
A tire application category for mixed highway and local operation — typically including city pickup and delivery, moderate highway mileage, varied loads, and more frequent stops than long-haul service. Regional tires often balance tread depth, void ratio, and tread pattern for both highway efficiency and local-service traction demands.
The practice of ensuring that tires in dual assemblies — or across an axle — are matched by size, load range, tread depth, and in some cases brand, to prevent differential wear and load distribution problems. Matching requirements are stricter for dual assemblies than for single positions because mismatched duals interact constantly while rolling.
A tire economics metric calculated as total tire cost divided by expected or actual removal mileage. A lower cost per mile indicates better overall value when comparing tire options. The calculation can include casing credit, mounting fees, and early removal penalties to produce a more complete picture than purchase price alone.
A managed system for purchasing, maintaining, tracking, and replacing tires across a commercial fleet. A structured fleet tire program typically includes standardized specifications, position-based pull policies, casing management, retread decisions, supplier relationships, and data collection on removal reasons and mileage. Programs reduce per-tire cost and improve fleet safety consistency.