Tires and Wheels
7 LessonsTire construction, wear patterns, TPMS diagnosis, and wheel service best practices.
Overview
Tires are the only thing connecting the vehicle to the road. This module covers tire construction, load ratings, speed ratings, wear patterns, rotation patterns, TPMS systems, wheel balancing, and the knowledge needed to sell tires and wheel services confidently.
Lessons
LESSON 01
Tire Basics
Tires are the only part of the vehicle that touches the road. Everything — acceleration, braking, steering, handling — happens through four contact patches each roughly the size of your hand. Understanding tires is understanding the foundation of vehicle dynamics.
Reading a tire size
A tire marked P225/65R17 breaks down like this. P means passenger vehicle. 225 is the section width in millimeters — the tire is 225mm wide from sidewall to sidewall. 65 is the aspect ratio — the sidewall height is 65 percent of the section width. R means radial construction. 17 is the wheel diameter in inches. Following the size you will find a load index number and a speed rating letter. The load index tells you the maximum weight each tire can support. The speed rating tells you the maximum sustained speed the tire is designed for. Never install tires with a lower load index or speed rating than the vehicle requires.
Tire pressure
The correct tire pressure is on the placard on the driver door jamb — not on the tire sidewall. The number on the sidewall is the maximum pressure the tire can handle, not the recommended operating pressure. Underinflation causes the outer edges of the tread to carry most of the load, wearing the shoulders faster and generating excess heat. Overinflation puts all the load on the center of the tread, wearing the center faster and reducing the contact patch. Both reduce tire life and compromise handling.
TPMS — Tire Pressure Monitoring System
Federal law requires TPMS on all vehicles sold after 2007. Direct TPMS uses a pressure sensor inside each tire that transmits wirelessly to the BCM. Indirect TPMS uses the ABS wheel speed sensors to detect a low tire by comparing rotational speeds — a low tire has a slightly smaller diameter and rotates faster. After any tire service, TPMS sensors must be relearned to the correct wheel positions. Always check whether the vehicle uses direct or indirect TPMS before performing tire service.
LESSON 02
Tire Wear Patterns
Tire wear patterns are diagnostic tools. The way a tire wears tells you exactly what is wrong with the vehicle before you put it on a rack. Learn to read them and you can diagnose alignment, suspension, and inflation problems from the parking lot.
Center wear
Excessive wear down the center of the tread with the shoulders still showing good tread depth means overinflation. The tire is ballooned outward and only the center contacts the road. Reduce pressure to the placard specification.
Edge wear — both shoulders
Both shoulders worn evenly with the center still showing tread means underinflation. The tire is sagging under the vehicle weight and the edges carry the load. This is one of the most common wear patterns because most people do not check tire pressure regularly.
One-sided wear
One shoulder worn significantly more than the other side indicates a camber problem. Inside edge wear is excessive negative camber. Outside edge wear is excessive positive camber. A worn ball joint, worn strut mount, bent strut, or bent knuckle can all cause camber to shift on one corner. Align first, but investigate why camber is out — it does not change on its own without a worn or damaged component.
Feathering or sawtooth wear
Run your hand across the tread — if it feels smooth in one direction and rough or jagged in the other, the tire is feathered. Feathering is caused by incorrect toe alignment. This is the fastest wear pattern — incorrect toe can destroy a set of tires in 10,000 miles. Check and correct toe immediately. Feathering almost always indicates worn tie rod ends that allowed toe to drift.
Cupping or scalloping
Dips or cups worn into the tread surface in an irregular wavy pattern indicate worn shock absorbers or struts. The tire is bouncing against the road surface instead of maintaining steady contact. Replace the worn dampers and the new tires will wear evenly.
LESSON 03
Wheel Balancing and Rotation
Wheel balance
A perfectly round tire mounted on a perfectly round wheel would roll without any vibration at any speed. In reality, every tire and wheel combination has slight weight imbalances. A heavy spot as small as half an ounce creates a vibration that increases with speed. A wheel balancer spins the assembly and identifies where to place counterweights to equalize the mass distribution. A vibration felt in the steering wheel at highway speed that was not there before and started after a tire service almost always means the front wheels were not balanced correctly.
Static vs dynamic balance
Static imbalance means the heavy spot is on one plane — the tire bounces up and down as it rotates. Dynamic imbalance means the heavy spot is off-center — the tire wobbles side to side. Modern spin balancers detect both simultaneously and place weights on both the inner and outer rim flanges to correct both types at once. Always use a dynamic balance on all four wheels.
Tire rotation
Rotating tires moves each tire to a different position on the vehicle at regular intervals — typically every 5,000 to 7,500 miles. The purpose is to equalize wear across all four tires because front and rear tires wear differently. Front tires on a FWD vehicle wear faster because they handle driving, steering, and most of the braking simultaneously. The standard rotation pattern for non-directional tires crosses the rears to the front and moves the fronts straight back. Directional tires — tires with a tread pattern designed to rotate in only one direction — can only be swapped front to rear on the same side.
LESSON 04
Tire Construction
A tire looks simple from the outside — black rubber in a round shape. But inside, a tire is an engineered structure with multiple layers, each doing a specific job. Understanding how a tire is built explains why tires fail, why some ride better than others, and why the markings on the sidewall matter.
Radial vs bias ply
Every tire on a modern passenger vehicle is a radial tire. The plies — layers of fabric cords embedded in rubber — run radially, straight across from bead to bead, perpendicular to the direction of travel. Think of the cords running from the left sidewall across the bottom to the right sidewall like the rungs of a ladder. This allows the sidewall to flex independently of the tread. The result is better road contact, cooler running temperatures, and longer tread life. Bias ply tires have plies that run diagonally — at an angle — crossing each other in layers. The sidewall and tread are structurally linked, which makes the tire stiffer and hotter. Bias ply is still used on some trailers and off-road equipment but has been replaced by radial on every passenger vehicle and truck.
What the belts do
Under the tread surface, steel belts wrap around the circumference of the tire. These belts stabilize the tread and keep it flat against the road under load and at speed. Without belts, the tread would distort and squirm under cornering and braking forces. A shifted belt — where one belt layer moves relative to another — causes a vibration or pull that cannot be balanced out. A tire with a shifted belt must be replaced. You can sometimes spot a shifted belt by looking at the tread — one section may appear slightly raised or the tread pattern may look uneven.
Run-flat technology
Run-flat tires have reinforced sidewalls that can support the vehicle weight even with zero air pressure. The extra-stiff sidewall prevents the tire from collapsing when punctured. Most run-flats allow you to drive 50 miles at up to 50 mph after a complete loss of pressure — enough to reach a service location. The trade-off is a firmer ride because the sidewall is inherently stiffer. Run-flat tires require TPMS because you cannot feel the difference in handling when the tire goes flat — the reinforced sidewall hides the pressure loss. Vehicles designed for run-flats often have no spare tire.
Decoding the sidewall — P215/65R16 explained
P — passenger vehicle tire. LT would mean light truck, which has a heavier construction. 215 — the section width in millimeters, measured from sidewall to sidewall when mounted and inflated. 65 — the aspect ratio. The sidewall height is 65 percent of the section width. A lower number means a shorter sidewall — a 45 series tire has a much shorter sidewall than a 65, which gives sportier handling but a harsher ride. R — radial construction. 16 — the wheel diameter in inches that this tire fits. After the size you will find numbers and letters like 95H. The 95 is the load index — it corresponds to a specific weight capacity per tire from a standardized chart. The H is the speed rating — the maximum sustained speed the tire is engineered to handle safely.
LESSON 05
Speed Ratings and Load Index
After the tire size on every sidewall, you will find a number and a letter — something like 95H or 102T. These are not decoration. They tell you the maximum weight each tire can carry and the maximum speed the tire is designed to sustain safely. Installing tires with the wrong rating can create a dangerous situation that the driver never sees coming until something fails.
Load index — the number
The load index is a number that corresponds to a weight capacity from a standardized chart. For example, a load index of 91 means each tire can carry 1,356 pounds. A load index of 95 means 1,521 pounds per tire. A load index of 105 means 2,039 pounds per tire. Multiply by four and that is the total weight the tires can support. The vehicle manufacturer specifies a minimum load index for the vehicle. Never install a tire with a lower load index than specified. On a pickup truck or SUV that carries heavy loads or tows, the load index is especially critical. An underrated tire carrying more weight than it was designed for generates excessive heat, which breaks down the internal structure and leads to a blowout.
Speed rating — the letter
The speed rating letter tells you the maximum sustained speed the tire is designed for. Common ratings: S is 112 mph, T is 118 mph, H is 130 mph, V is 149 mph, W is 168 mph, Y is 186 mph. The rating is not about how fast you drive — it is about the tire's internal construction and heat dissipation capability at speed. A higher speed rating means the tire is built with materials and construction that handle the heat and centrifugal forces generated at higher speeds. Downgrading the speed rating means the tire may not handle sustained highway driving as well, especially in hot weather or under load.
What happens with the wrong rating
Installing a tire with too low a load index on a vehicle that carries heavy loads results in excessive sidewall flex, heat buildup, and potential blowout. Installing a tire with too low a speed rating on a vehicle that regularly drives at highway speeds risks tire failure from heat and structural fatigue. The tire may look fine and hold air perfectly, but the internal construction is being stressed beyond its design capability. Always match or exceed the original equipment load index and speed rating. The information is on the door placard and in the owner manual.
Never downgrade load index or speed rating below the vehicle manufacturer specification. A tire that looks fine on the outside can be failing internally from overload or heat stress. Tire failure at highway speed is catastrophic — the driver loses control instantly.
LESSON 06
Tire Aging and Date Codes
Here is something most people never think about. A tire with perfect tread depth can still be unsafe. Rubber deteriorates with age regardless of use. A tire that has been sitting in a warehouse or mounted on a vehicle that does not get driven much will look fine on the surface while the internal rubber compounds have hardened, cracked, and lost their ability to flex safely. Age kills tires just as surely as wear does.
How rubber ages
Rubber is a polymer. Over time, oxygen breaks down the molecular chains in a process called oxidation. Heat accelerates it. UV exposure accelerates it. Even without those factors, the rubber slowly degrades from the inside out. The oils that keep the rubber flexible evaporate. The compound becomes brittle. Micro-cracks form in the sidewall and between the internal plies. These cracks may not be visible on the surface until the tire is well past the point of safe use. A tire that has aged out will not grip the road the way it did when new, and it is far more susceptible to sudden failure — a blowout from a pothole or road debris that a fresh tire would shrug off.
Reading the DOT date code
Every tire manufactured for sale in the United States has a DOT code stamped into the sidewall. The last four digits of the DOT code are the manufacture date. The first two digits are the week. The last two digits are the year. A code ending in 2319 means the tire was manufactured in the 23rd week of 2019. The DOT code is usually on only one side of the tire — if you do not see it on the outboard sidewall, check the inboard side. On tires made before the year 2000, the date code is only three digits, but you are unlikely to encounter those in service unless they have been sitting in storage for decades.
The six-year rule
Most tire manufacturers and safety organizations recommend replacing tires that are six years old from the date of manufacture, regardless of remaining tread depth. Some manufacturers extend that to ten years as the absolute maximum. After six years, inspect the tire carefully for sidewall cracking, hardening of the rubber, and any signs of deterioration. After ten years, replace the tires regardless of condition or appearance. This applies to the spare tire too — the spare sitting under the truck bed or in the trunk ages at the same rate as the tires on the ground. A customer who has a blowout on a 10-year-old spare with full tread depth learned this lesson the hard way.
Always check the DOT date code when installing used tires or when a customer brings in tires they purchased elsewhere. A tire with full tread but a manufacture date older than six years should be flagged and the customer informed of the age-related risk. Never install a tire older than ten years regardless of appearance.
LESSON 07
TPMS Systems
The Tire Pressure Monitoring System — TPMS — is a federally mandated safety system on all vehicles sold in the US after 2007. Its job is simple: warn the driver when a tire drops below a safe pressure. The TPMS light on the dash means at least one tire is 25 percent or more below the placard pressure. That is significant — a tire rated at 35 PSI triggering at around 26 PSI is already dangerously low and generating excess heat.
Direct TPMS
Direct systems use a physical sensor mounted inside each tire, usually attached to the valve stem or banded to the inside of the wheel. Each sensor contains a pressure transducer, a temperature sensor, a battery, and a small radio transmitter. The sensor reads the actual tire pressure and wirelessly transmits that data to a receiver module — usually the BCM or a dedicated TPMS module. The system knows the exact pressure in each tire and can display individual tire pressures on some vehicles. When any sensor reads below the threshold, the TPMS warning light illuminates.
Indirect TPMS
Indirect systems do not use pressure sensors inside the tires. Instead, they use the existing ABS wheel speed sensors to detect pressure loss. The principle is simple — a tire with lower pressure has a slightly smaller rolling diameter and rotates faster than the other tires. The ABS module compares the rotational speeds of all four wheels. If one wheel consistently spins faster than the others, the system flags it as low pressure. Indirect TPMS is less precise than direct. It cannot tell you the actual pressure — only that one tire is significantly lower than the others. It also cannot detect a situation where all four tires are equally low.
Sensor battery life
Direct TPMS sensors run on a small lithium battery sealed inside the sensor housing. The battery is not replaceable — when it dies, you replace the entire sensor. Battery life is typically 7 to 10 years depending on the sensor design and how frequently it transmits. As sensors age, the battery weakens and transmission range decreases. A sensor with a dying battery may intermittently fail to communicate with the module, causing the TPMS light to flash for 60 to 90 seconds at startup — which indicates a system fault, not a low tire — and then stay solid. When one sensor battery dies, the others are usually close behind because they were all installed at the same time.
The relearn procedure
Each TPMS sensor has a unique ID. The TPMS module must know which sensor ID belongs to which wheel position — left front, right front, left rear, right rear. Any time tires are rotated, replaced, or sensors are replaced, the system needs to relearn which sensor is where. Some vehicles use a scan tool to program each sensor ID to a position. Some use a TPMS activation tool — a handheld device that sends a signal to wake up each sensor one at a time while the module listens. Some vehicles self-learn by driving at a specified speed for a specified distance after a reset. Always look up the specific relearn procedure for the vehicle. Skipping the relearn means the dash may display the wrong pressure for each tire position or the TPMS light may stay on.
Why the light comes on
Solid TPMS light — at least one tire is below the pressure threshold. Check and correct all tire pressures to the door placard specification. The light should turn off after a few minutes of driving. If it does not, one tire may have a slow leak or the sensor may be reading incorrectly. Flashing TPMS light at startup that then turns solid — this indicates a system fault, not a tire pressure issue. A sensor battery has died, a sensor is not communicating, or the relearn was not performed after a tire service. Scan the TPMS module for codes to identify which sensor is faulting.
Key Components
- Tire construction and ratings
- Tire Pressure Monitoring System (TPMS)
- Wheel types and specifications
- Lug nuts and torque specs
- Valve stems and sensors
How It Works
Tires are engineered composites of rubber, steel, and fabric designed to grip the road, absorb impacts, support vehicle weight, and last tens of thousands of miles. TPMS sensors mounted in each wheel monitor pressure and alert the driver when pressure drops below the threshold.
Common Problems
- Uneven tire wear from alignment issues
- TPMS sensor battery failure
- Wheel vibration from balance or bent wheel
- Tire pressure loss from bead seat corrosion
- Wrong torque on lug nuts causing warped rotors
Diagnostic Tips
- Read wear patterns: inside edge = too much negative camber, center = overinflation
- TPMS relearn procedure varies by manufacturer
- Always torque lug nuts in a star pattern to spec
- Check runout on the wheel if vibration persists after balance
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