Tire Service, Balance & TPMS

Tires are the first job most apprentices see end-to-end. They look simple. They are not. A lug nut torqued with an air gun at 180 ft-lbs on a 100 ft-lbs spec will stretch a wheel stud until it snaps three weeks later on the highway. A valve stem over-torqued with channel locks will crack at the base and leak six months after you sent the car out. A TPMS relight that never got reset is a callback and a one-star review.

This is the full mount-and-balance walkthrough.

Tire Sizing Code

A sidewall reads something like 225/60R17 98V. Break it down:

| Element | Value | Meaning                                         |
|---------|-------|-------------------------------------------------|
| 225     | 225   | Section width in millimeters                    |
| 60      | 60    | Aspect ratio: sidewall height is 60% of 225 mm  |
| R       | R     | Radial construction                             |
| 17      | 17    | Rim diameter in inches                          |
| 98      | 98    | Load index, 1,653 lb per tire                   |
| V       | V     | Speed rating, 149 mph sustained                 |

Do not drop below original load index on anything that tows, hauls, or carries passengers regularly. Do not drop below original speed rating on a sport sedan that was designed for a V or W tire. You match or exceed. Never undercut.

Load Index Table

| Index | lb per tire |
|-------|-------------|
| 85    | 1,135       |
| 89    | 1,279       |
| 91    | 1,356       |
| 94    | 1,477       |
| 98    | 1,653       |
| 102   | 1,874       |
| 105   | 2,039       |
| 109   | 2,271       |
| 113   | 2,535       |

Speed Rating Table

| Symbol | Max sustained |
|--------|---------------|
| S      | 112 mph       |
| T      | 118 mph       |
| H      | 130 mph       |
| V      | 149 mph       |
| W      | 168 mph       |
| Y      | 186 mph       |
| (Y)    | above 186 mph |

DOT Date Code

The DOT code ends in a 4-digit date: week-week-year-year. "2423" means the 24th week of 2023. Industry rules of thumb:

• 6 years: conservative replacement age regardless of tread
• 10 years: hard stop, even on a low-mile spare
• Any tire with sidewall cracking (weather checking) goes, date aside

Spares and trailer tires are the biggest offenders here. A 12-year-old spare will blow within a mile of being mounted at highway speed.

Tread Depth

| Tread depth | Condition                              |
|-------------|----------------------------------------|
| 10/32"+     | New (most all-seasons start 10-11/32") |
| 6/32"       | Halfway gone, fine                     |
| 4/32"       | Marginal in the wet                    |
| 3/32"       | Recommend replacement                  |
| 2/32"       | Legal minimum, already failing         |

Use a depth gauge, not a penny. Match tire wear across an axle within 2/32". On all-wheel-drive cars, within 2/32" across all four is often a manufacturer requirement to protect the transfer case and center differential.

Dismount Procedure

1. Let the air out via the valve core. Do not just pull the core with your thumb on a hot tire.
2. Break the bead with the bead breaker, not by pounding with a tire iron. Pinch wounds on the inner liner lead to slow leaks.
3. Position the demount head to slip over the bead. Use bead lube generously on both beads.
4. Rotate the tire off. Flip and repeat for the bottom bead.

Do not skip bead lube. A dry dismount scuffs the bead and creates leak paths on the next mount.

Mount Procedure

1. Inspect the rim. Look for cracks, bent flange, deep curb rash at the bead seat, and corrosion. A bent flange gets replaced. A cracked rim is scrap. Do not remount on damaged rims.
2. Replace the valve stem every time you dismount. TR-413 snap-in for steel, metal clamp-in for most alloy wheels, sensor-stem for TPMS.
3. Lube both beads.
4. Mount the tire paying attention to direction arrows and outside sidewall callout.
5. Inflate in a bead-seating cage or with the tire on the machine. Pop the beads with controlled inflation.

Safety rule: OSHA and every tire-industry bulletin says to never exceed 40 psi to seat a bead. If it does not seat by 40 psi, something is wrong, most likely bead lube, a bent rim, or a stretched bead. Never stand in the plane of the tire while inflating. Sidewall failures at seating are rare but take faces off.

Lug Nut Torque

Air-gun-tight is a myth. Torque with a calibrated torque wrench, or a torque stick rated to the spec plus final check with a click-type wrench.

| Vehicle class                          | Typical spec    |
|----------------------------------------|-----------------|
| Small passenger car                    | 80-90 ft-lbs    |
| Midsize passenger car / crossover      | 90-100 ft-lbs   |
| Full-size pickup, half-ton             | 120-140 ft-lbs  |
| Three-quarter / one-ton pickup         | 140-175 ft-lbs  |
| Many Teslas                            | 129 ft-lbs      |
| Many BMW / Mercedes (wheel bolts)      | 88-103 ft-lbs   |

Always look up the spec. Tighten in a star pattern. Final pass at spec with a torque wrench. Apply the retorque sticker: "Return in 50-100 miles for a free retorque." This protects you and the customer when an alloy wheel relaxes on its seat.

Wheel Balance

Imbalance types:

• Static imbalance: heavy spot concentrated at one point, causes bounce
• Dynamic imbalance: heavy spots offset front and rear of the tire centerline, causes shimmy
• Road-force imbalance: tire high-spot matched against rim low-spot to minimize radial run-out

Most shop balancers are dynamic. Road-force balancers (Hunter GSP9700 and similar) are standard at dealerships and heavy on comebacks. Always clean the mounting face and the center bore before a balance. Wheel weight types:

• Clip-on weights on steel wheels, clip matched to the rim flange shape
• Adhesive tape weights for alloys, hidden behind the spokes where possible
• Never stack dissimilar weight types unless the machine asks for it

Common cause of a post-balance comeback:

• Debris or old weight residue inside the rim
• Bent or out-of-round rim
• Cupped or out-of-round tire (feathered or unevenly worn)
• Dirty mounting face
• Loose spare in the trunk (yes, really)

TPMS

Two types:

• Direct: a pressure sensor with a battery inside each wheel, broadcasting to the BCM. Battery life 5 to 10 years.
• Indirect: the BCM infers low pressure from wheel-speed differences via the ABS sensors. No in-wheel hardware.

Relearn procedures vary by OEM:

| Brand                    | Typical relearn                                              |
|--------------------------|--------------------------------------------------------------|
| GM                       | OBD-II auto-relearn with pressure decrease then re-inflation |
| Ford                     | Magnet or TPMS tool at each wheel, in sequence               |
| Toyota / Lexus           | "Set" button, then drive at speed, or TPMS tool              |
| Honda                    | Infotainment menu or drive relearn                           |
| BMW                      | iDrive menu reset, then drive-cycle, may require scan tool   |
| Dodge / Chrysler         | TPMS tool at each wheel                                      |
| Most European, post-2014 | Scan tool or OBD-II TPMS menu                                |

If a sensor is dead, replace the full stem assembly. Clone the ID from the old sensor with a TPMS tool when possible, or register the new sensor ID to the vehicle. Never disable the warning light and call it done. Never reuse a sensor nut, grommet, or seal kit: every sensor comes with fresh rubber and you use it.

Valve-stem torque on metal TPMS stems: 4 to 8 in-lbs. Over-torque cracks the stem at the base and you get a slow leak three months later. Use an in-lb torque tool.

Leak Diagnosis

The hunt order:

1. Inflate to spec, spray soapy water at the valve core, bead, stem base, and all the way around the sidewall and tread.
2. Watch for bubbles. Big foam is a large leak, fine fizz is a slow leak.
3. Check the valve core with a valve-core torque tool. Cores fail silently.
4. Pull the tire and look at the inner liner under light. Road-hazard punctures often show their exit point on the inside, not the outside.
5. Inspect the rim bead seat for corrosion pitting, a classic aluminum-wheel slow-leak cause. Wire-brush and reseat with a bead sealer.

Repair vs Replace

Industry (RMA / USTMA) guidance:

• Repairable: through-the-tread punctures, in the middle 75% of the tread, 1/4 inch diameter or less.
• Not repairable: sidewall or shoulder punctures, cuts exceeding 1/4 inch, repairs that overlap an old repair, any tire run flat enough to damage the inner liner.
• Plug-only from the outside is not a permanent repair. Combination plug-patch (mushroom patch) from the inside after buffing the liner is the correct method.

Always dismount, inspect the inner liner for run-flat damage, and repair from inside.

Post-Service Ritual

• Torque all four wheels to spec with a click-type wrench.
• Reset TPMS per the OEM procedure and verify the light goes out.
• Verify all four at the door-placard PSI, not the max sidewall PSI.
• Check the spare at its own spec pressure.
• Put the retorque sticker on the steering wheel or driver window.
• Clean the fender, sill, and seat. A greasy seat cushion is a customer complaint the advisor cannot fix.

That is the tire bay. Quality at the last four items is what separates an apprentice you keep from one you replace.