Tires are the highest-volume work in most shops and the most commonly rushed. They’re also one of the best diagnostic tools you have — a worn tire is a written record of everything wrong with the suspension, alignment and inflation habits above it.
Here’s what’s worth knowing properly, whether you’re fitting them or selling them.
Reading the sidewall
Take a common size like P225/65R17 102H:
- P — passenger vehicle (LT would be light truck)
- 225 — section width in millimetres
- 65 — aspect ratio: sidewall height as a percentage of width
- R — radial construction
- 17 — rim diameter in inches
- 102 — load index
- H — speed rating
The last two are the ones people get careless with. Fitting a lower load index or speed rating than the vehicle calls for changes how the car handles and how the tire copes with heat — and it’s the sort of thing that gets examined closely after an incident.
The DOT date code
The last four digits of the DOT code are the week and year of manufacture — 3224 means the 32nd week of 2024.
This matters because rubber ages whether the tire is used or not. Manufacturers commonly recommend replacement somewhere in the six-to-ten year window regardless of remaining tread. A customer with a “nearly new” spare from 2011 does not have a usable spare.
Tread depth
Two thirty-seconds of an inch is the legal minimum in most states. It is not a safe target — wet braking and aquaplaning resistance fall off long before that.
Rough guide: around 4/32″ is where wet performance is noticeably compromised, and it’s an honest point to start the conversation rather than waiting for the legal limit.
Wear patterns tell you what’s wrong
This is the diagnostic part. Learn to read these and you’ll catch faults before anyone complains:
- Both edges worn, centre fine — chronic underinflation.
- Centre worn, edges fine — chronic overinflation.
- One edge only — camber, or a bent component.
- Feathered edges you can feel running your hand across the tread — toe.
- Cupping or scalloping — worn shocks or struts, or a persistent imbalance.
- Flat spots — lock-up braking, or a vehicle parked for a long period.
Always run your hand across the tread, not just look at it. Feathering is much easier to feel than to see.
TPMS
Sensors have batteries and they die — typically after several years, and they aren’t serviceable. Every tire job on a TPMS-equipped vehicle should include the service kit: valve core, seal, nut and cap. Reusing corroded hardware is how a slow leak becomes a comeback.
Relearn procedures vary widely by manufacturer. Check before you start rather than after the light stays on.
Torque properly, and mention the retorque
Use a torque wrench, tighten in a star pattern, and torque to the vehicle’s specification — not the impact gun’s. Over-torquing warps rotors and stretches studs; under-torquing is considerably worse.
Advise the customer to return for a retorque after the first 50 to 100 miles. Most won’t, but saying it is both good practice and good documentation.
Balancing and matched sets
Standard spin balancing handles most vibration complaints. Persistent vibration after a good balance usually means a tire with an internal problem, and that’s what road force balancing exists to find.
On all-wheel-drive vehicles, mismatched rolling circumference between axles stresses the driveline. Many manufacturers specify replacing all four, or matching tread depth within a tight tolerance. Check the specification before selling two.
If you’re selling tires
Customers assume tires are a commodity and that you’re upselling. The way through that isn’t pressure — it’s showing them. Bring them to the car, run their hand across the feathering, show them the tread gauge, show them the date code on a tire that looks fine and is nine years old.
People buy what they can see. And the tires you sold because someone understood the problem don’t come back as complaints.
Working with tires all day
Tire and alignment work is a large share of what our shops do, and it’s one of the better places to start if you’re newer to the trade — high volume, quick feedback, and a natural path into suspension and steering diagnostics.
We hire technicians at every skill level across our Midas shops in Wayne, Oakland and Washtenaw counties.