Tire Terms, Explained in Plain Words

Reading the sidewall

Tire size: the string such as P225/60R17 molded into the sidewall. It names the width, aspect ratio, construction, and wheel diameter. It is the first thing we ask for on the call because it decides what fits.

P, LT, and ST prefix: the letter or letters before the size. P means passenger car, LT means light truck, and ST means special trailer. An ST tire belongs on a trailer only, which matters when we help with trailer tires where available.

Section width: the first number, the width of the tire in millimetres. 225 means 225 millimetres across the widest part of the tire.

Aspect ratio: the number after the slash, the sidewall height as a percentage of the width. A lower number means a shorter, stiffer sidewall.

R: radial construction, the way nearly every modern car tire is built. The plies run across the tire from bead to bead.

Wheel diameter: the last number in the size, the rim size in inches. A 17 goes on a 17-inch wheel and nothing else.

Load index: the number after the size, for example 98. It stands for the maximum weight one tire can carry at its rated pressure. A replacement should meet or exceed the number on the placard.

Speed rating: the letter after the load index, for example H. It is the top speed the tire is built to sustain. Match it to the placard rather than guessing.

Service description: the load index and speed rating together, such as 98H. Technicians read them as one unit.

DOT code: the run of letters and numbers after the letters DOT. It identifies the plant, size, and batch. The last four digits are the date.

Tire date code: the last four digits of the DOT code, week then year. 2322 is the 23rd week of 2022. Many manufacturers advise an inspection after six years and replacement by ten, whatever the tread looks like.

Placard: the sticker on the driver's door jamb. It lists the original tire size and the correct cold pressure for front and rear. When we set pressure on site, this is the number we use.

Max pressure: the pressure molded on the sidewall. It is the most the tire can safely hold, not the amount to run. The placard number is lower and is the right one.

Cold pressure: the pressure in a tire that has not been driven for a few hours. Every placard and every gauge reading assumes a cold tire, because driving heats the air and raises the number.

M+S: mud and snow, a mark that the tread pattern is designed for some mud and light snow. It describes the tread shape. It is not a winter tire rating.

Parts of a tire

Tread: the patterned rubber that touches the road. It is the only part of a tire where U.S. Tire Manufacturers Association repair procedures allow a repair.

Shoulder: the rounded edge where the tread meets the sidewall. Damage here is not repairable because the tire flexes too much in that spot.

Sidewall: the flat face between the shoulder and the bead. It carries the markings and flexes with every turn of the wheel. A puncture, cut, or bulge here means replacement.

Bead: the stiff, wire-reinforced inner edge that seals against the wheel. When a technician dismounts a tire, the bead is what comes off the rim.

Inner liner: the airtight rubber layer inside a tubeless tire. A patch is applied here, from the inside, which is why a proper repair means taking the tire off the wheel.

Belts and plies: the layers of cord and steel under the tread and in the sidewall. They give the tire its shape and strength. A tread puncture goes through the belts, and the repair seals the hole in them and in the liner.

Valve stem: the short rubber or metal stem you add air through. It can crack with age or leak at its core. Valve stem fixes are something we do on site.

Valve core: the small spring-loaded insert inside the stem. A loose or dirty core is a common slow leak and a quick fix.

TPMS sensor: the small sensor inside the wheel, usually attached to the valve stem, that reports pressure to the dash on direct systems. Replacing one is not a listed service. Ask on the call.

Wheel and rim: the wheel is the metal part the tire mounts on, and the rim is its outer edge where the bead seats. Corrosion or a bend on the rim is a common cause of a bead leak.

Damage and failure

Puncture: a hole through the tread from a nail, screw, or other object. In the tread area and within repair limits, this is the classic on-site repair.

Slow leak: air loss over days or weeks rather than minutes. The pressure light returns after every fill. Finding it means checking the tread, the valve, and the bead in turn.

Bead leak: air escaping between the bead and the rim, often from corrosion, dirt, or a bent rim. It is fixed by dismounting, cleaning or sealing the seat, and remounting.

Valve stem leak: air escaping at the stem or its core. Often the simplest leak to fix and one we handle at the car.

Sidewall bulge: a bubble on the sidewall where the inner layers have separated, usually after a pothole or a curb. The tire can fail without warning. It is a replacement, and our sidewall damage guide explains why.

Blowout: sudden, total loss of air, often with a bang and a pull to one side. Keep the wheel straight, ease off the gas, and get to the shoulder. The tire is a replacement.

Run-flat damage: a run-flat tire driven after losing pressure has been used past its design, even if it looks fine. Most manufacturers call for replacement rather than repair.

Dry rot: fine cracks in the sidewall or between tread blocks from age, sun, and sitting. Common on spares and trailer tires. A dry-rotted tire is a replacement, not a repair.

Tread separation: the tread peeling away from the belts, felt as a thump that grows with speed. Stop driving. The tire is a replacement.

Cupping: scalloped, uneven wear around the tread, usually from worn suspension or a balance problem. It is a sign for a shop, not something a roadside visit fixes.

Driven flat: a tire that was rolled on with no air. The sidewall grinds against itself and the damage is inside. Industry guidance calls for an inside inspection before any repair, and often the answer is replacement.

Repair terms

Plug: a sticky rubber strip pushed into the puncture from the outside without removing the tire. It fills the hole but does not seal the inner liner. Tire Industry Association guidance treats a plug alone as a temporary fix.

Patch: a rubber patch bonded to the inner liner from the inside. It seals the liner but leaves the puncture channel open, so water can reach the belts.

Plug-patch: a one-piece repair with a stem that fills the hole and a patch that seals the liner, applied from the inside. It is the repair the industry treats as proper for a tread puncture. It is what we mean by repairing a tire on site when it qualifies.

Repairable area: the central tread, away from the shoulder, where a puncture can be repaired. Outside that area the tire flexes too much for a repair to hold.

Repair size limit: the maximum puncture diameter industry guidelines allow a repair for. A larger hole, a cut, or two punctures close together push the tire to replacement. The technician measures it at the car.

Dismount and remount: taking the tire off the wheel and putting it back. A proper plug-patch needs it, because the work is done from the inside.

Torque: how tight the lug nuts are. They are tightened to the maker's figure so the wheel seats evenly and stays put. Too loose is dangerous, and too tight can damage the studs or the brake rotor.

Star pattern: the crisscross order for tightening lug nuts, across the wheel rather than around it. It is how a wheel goes back on straight.

Balancing: adding small weights to the wheel so it spins without wobble. It needs a balancing machine, so it is shop work and not a stated on-site service. Ask on the call if you need it.

Alignment: adjusting the angles of the wheels so the car tracks straight and the tires wear evenly. Shop only.

Sealant: the liquid in an inflator kit or an aerosol can that coats the inside of the tire to stop a small leak. It is temporary, can affect the TPMS sensor, and many shops will not repair a tire that has had sealant in it. Tell us if you used it.

Spares and roadside terms

Full-size spare: a fifth tire the same size as the other four. It can be driven like a normal tire, though it may be older and lower on air than you expect. Check its pressure and its date.

Compact spare: the small, narrow spare, often called a donut. It has a speed limit and a distance limit printed on the tire. It is a bridge to a repair or replacement, not a fix.

Inflator kit: the compressor and sealant bottle that replaces the spare in many newer cars. It works only on small tread punctures and does nothing for sidewall damage or a blowout.

Run-flat: a tire with stiff sidewalls that can be driven a limited distance at reduced speed after losing pressure. The limit is set by the tire maker and printed in the owner's manual. Sidewall marks vary by brand, such as RFT, ROF, ZP, SSR, or DSST.

Mobile tire repair: the technician comes to the vehicle and repairs or replaces the tire where it sits. Driveways, lots, workplaces, hotels, and highway shoulders where it is safe to stop.

Tow: moving the car on a truck to a shop. The right call when the car cannot be safely worked on where it is, the wheel is damaged, or no tire is available.

Dispatch: the moment we send a technician to you. Pricing is explained before any work begins, and a cancellation after a technician has been dispatched may carry a trip fee.

ETA: estimated time of arrival. We give you an honest one on the call based on your location, traffic, and technician availability. It is an estimate, not a promise.

Estimate: the price we explain before any work begins. Quotes are estimates until the technician sees the tire, because damage location and tire condition can change the plan.