Tire care & rotation

Four things decide how long a set lasts, and three of them are free. This is the whole list — there is no fifth thing being withheld.

Pressure

The single highest-return habit, and the most neglected. An under-inflated tire flexes more in the sidewall, runs hotter, wears its shoulders, costs fuel, and in the worst case fails at speed.

• Check monthly, cold. Cold means parked three hours, or driven less than a mile. A warm tire reads several psi high and will have you letting air out of a correctly inflated tire.
• Use the door placard, not the sidewall. The number moulded into the tire is its maximum, not its recommended pressure. The placard in the driver's door jamb is what the vehicle was engineered around.
• Expect to add air as it gets colder. Pressure falls roughly 1 psi for every 10°F drop. The light coming on during the first cold snap is physics, not a leak.

Rotation

Tires wear at different rates by position — fronts carry more weight and do the steering, and on a front-wheel-drive car they do the driving too. Moving them around evens it out, and an even set is a set you replace all at once instead of twice.

Every 5,000–8,000 miles, or every other oil change. The pattern depends on the car:

• Front-wheel drive: fronts go straight back; rears cross to the opposite front.
• Rear-wheel and four-wheel drive: rears go straight forward; fronts cross to the opposite rear.
• Directional tires (arrows on the sidewall) only move front to back on the same side. Crossing them puts the tread pattern backwards.
• Staggered fitments — different sizes front and rear — cannot be rotated front to back at all. Side to side only, and only if the tires are not directional.

Tread depth

New passenger tires start around 10/32". The legal minimum in most US states is 2/32", and that figure is a legal floor rather than a safe one.

• Below 4/32" wet stopping distances lengthen sharply and the risk of aquaplaning climbs. This is the point to start shopping.
• At 2/32" the moulded wear bars sit flush with the tread. The tire is done, whatever it looks like.

Two ways to check without a gauge. A quarter, Washington's head down: if the tread does not reach the top of his head you are below 4/32". A penny, Lincoln's head down: if you can see the top of his head you are at or below 2/32" and the tire is finished. Measure in three places across the tread and at several points around — uneven wear is itself the diagnosis.

Age

Rubber hardens over time whether or not the tire is used, and this is the part almost everybody misses. A tire with plenty of tread can still be unsafe because the compound has aged out — most commonly on a spare, a trailer, a classic, or a second car that sits.

The date is moulded into the sidewall as the last four digits of the DOT code: 3421 is week 34 of 2021. Most manufacturers say replace at six years regardless of tread, and at ten years it is not a judgement call.

What uneven wear is telling you

• Both shoulders worn, centre fine — chronic under-inflation.
• Centre worn, shoulders fine — over-inflation.
• One shoulder only — camber or alignment. A tire will not fix itself; the new one wears the same way.
• Scalloped or cupped patches — worn dampers or an out-of-balance assembly. Usually audible before it is visible.
• Feathered edges you can feel one way — toe misalignment.

All of these are worth an alignment check. Fitting a new set to a car that is out of alignment is buying the same wear pattern again at full price.

Replacing in pairs

If you are not replacing all four, replace in pairs and put the new pair on the rear — on any car, front- or rear-wheel drive. More grip at the front and less at the back means the rear lets go first in a wet corner, which is the hard failure to catch. It feels backwards and it is what every manufacturer recommends.

On all-wheel drive, check the manual before mixing at all: many systems require all four within a few 32nds of each other, and a mismatched set can damage a differential.