Everything you need to buy the right tire is molded into the side of the one you already have, and most of what shops argue about is settled by reading it. Here is how to read your own tires, what the services actually do, and where a tire can be repaired instead of replaced.
Reading the sidewall
Take a common size, 225/50R17 98V. It looks like a serial number and every part of it is a measurement.
- 225 is the section width in millimeters, across the widest part of the inflated tire. Wider is not automatically better: the width has to suit the wheel, and a tire stretched or squeezed onto the wrong rim width does not sit on the road the way it was designed to.
- 50 is the aspect ratio, the sidewall height as a percentage of the width, so this sidewall is 112 mm tall. A lower number means a shorter, stiffer sidewall, sharper steering and considerably less protection from a Michigan pothole.
- R is radial construction, which is essentially every passenger tire sold today.
- 17 is the rim diameter in inches, and the one figure with no flexibility in it at all.
- 98 is the load index, here 750 kg per tire. Trucks, vans and anything that tows need the index the manufacturer specified. A passenger-rated tire fitted to a van is a genuine safety problem rather than a technicality.
- V is the speed rating, here 149 mph. It is less about speed than about how the tire is built to shed heat, and it is worth matching.
The four digits most people never look at
At the end of the DOT code there is a four-digit number: the week and the year the tire was built. 2219 is the twenty-second week of 2019. Rubber hardens and crazes with age whether the car is driven or parked, so a tire with deep tread and a date code six or seven years old is still a tire to replace. We check the code on every tire that comes through, including the spare, because the spare is usually the oldest tire on the vehicle and the one you discover at the worst possible moment.
What the tread is telling you
The legal minimum is 2/32 of an inch, and wet stopping distance falls off well before that. In a Michigan winter, 4/32 is the honest point to start planning rather than the point at which you must act. The old penny test still works: Lincoln's head upside down in the groove, and if you can see the top of his head you are at or below 2/32.
The wear pattern is worth more than the depth, because it tells you why:
- Both shoulders worn, center fine. Chronic underinflation.
- Center worn, shoulders fine. Chronic overinflation.
- One shoulder only. Camber, which usually means something is bent or a bushing has collapsed rather than that the tire is bad.
- A feathered edge you can feel in one direction but not the other. Toe. This is the alignment angle that destroys tires fastest.
- Cupped or scalloped. Worn dampers letting the tire bounce.
Replacing a tire without fixing the pattern that killed it just puts a clock on the new one.
When a tire can be repaired
Often, and the industry rules are clear rather than a matter of opinion. A puncture can be repaired when it is in the tread area, not the shoulder and not the sidewall; when it is smaller than a quarter of an inch; and when the tire has not been run flat and damaged internally.
A proper repair is a patch-plug from the inside, with the tire dismounted so the inner liner can be inspected. A plug pushed in from the outside without taking the tire off is not a repair, it is a way to get home. We do it the first way, and if a tire cannot be safely repaired we will tell you why rather than default to selling you one.
Rotation, balancing and alignment are three different things
People use these interchangeably and they solve different problems.
Rotation moves the tires around the car so they wear evenly, because the front tires on most cars do the steering and most of the braking and therefore wear faster. Doing it on a sensible interval, usually with the oil change, is the cheapest way to get the most life out of a set.
Balancing corrects weight distribution around the wheel. An out-of-balance wheel produces a vibration that arrives at a particular speed, usually somewhere between 50 and 70 mph, and gets worse as the speed rises. Where a standard spin balance does not cure it, road force balancing measures the tire under load against a roller and finds stiffness variation inside the carcass, which is a different fault and one a normal balancer cannot see.
Alignment sets the angles the wheels sit at. It does not fix a vibration and balancing does not fix uneven wear. If somebody offers you one for the other's symptom, ask why.
All-season or winter, for a Michigan driver
Winter tires are a real upgrade and they are not necessary for everybody. The compound stays pliable below about forty-five degrees where an all-season hardens, and most of the difference comes from that rather than from the tread pattern.
If your route has hills, or you are out before the plows, they earn their money. If you drive cleared main roads in daylight, a good all-season with genuine tread depth is a defensible choice. What is not defensible is two winter tires on one axle and two all-seasons on the other, which gives you a car with two different levels of grip at either end.
One thing that is not optional: on an all-wheel-drive car, particularly a Subaru, all four tires need to be close in circumference. Replacing one new tire against three worn ones makes the center differential work continuously, and that is a genuine failure rather than a theoretical one. Replace in fours, or at minimum in matched pairs on the same axle.
What we do here
Mounting and balancing, road force balancing where a standard balance will not settle it, rotation, puncture repair done properly from the inside, TPMS sensor service, and new tires across the brands we can get quickly. Every tire that comes off gets its date code and wear pattern noted on the inspection report, so you get told what is coming rather than only what is due.