Tyre Pressure, Tread Depth, and the Numbers on Your Sidewall

Decoding the Numbers on Your Tyre Sidewall

That string of numbers and letters stamped into your tyre's sidewall — something like 225/55R17 97V — isn't random. Each segment carries specific information your vehicle depends on.

225

Section width in millimeters — how wide the tyre is from sidewall to sidewall.

55

Aspect ratio — the sidewall height expressed as a percentage of the section width. A lower number means a shorter, stiffer sidewall.

R

Radial construction — the internal ply cords run perpendicular to the direction of travel. Virtually all passenger tyres sold today are radial.

17

Rim diameter in inches — the wheel size the tyre is designed to fit.

97

Load index — a code indicating the maximum weight each tyre can safely carry. A 97 rating corresponds to roughly 1,609 lbs (730 kg) per tyre.

V

Speed rating — the maximum sustained speed the tyre is designed for. V indicates up to 149 mph (240 km/h). Never fit a tyre with a lower speed rating than your vehicle manufacturer specifies.

You'll also find a DOT code on the sidewall. The last four digits show the week and year of manufacture — for example, 2322 means the 23rd week of 2022. Tyres older than six years generally warrant professional inspection regardless of visible condition; most manufacturers recommend replacement by ten years from the manufacture date.

Tyre Pressure: Finding and Maintaining the Right PSI

The single most common — and consequential — tyre mistake families make is running on incorrect pressure. Underinflated tyres wear faster on the outer edges, reduce fuel economy, and can overheat at highway speeds. Overinflated tyres wear faster in the center and reduce the contact patch, compromising grip.

Where to Find Your Vehicle's Correct Pressure

The number you need is not the maximum PSI printed on the tyre itself — that's the tyre's structural limit, not your car's recommendation. Your vehicle's specified pressure is found on a sticker inside the driver's door jamb, in the owner's manual, or occasionally on the fuel-filler flap. Front and rear tyres sometimes require different pressures, so check both.

When and How to Check

• Always check pressure when tyres are cold — meaning the car has been parked for at least three hours or driven fewer than one mile. Driving heats the air inside, raising the reading by 4–8 PSI and giving a misleading result.
• Use a quality gauge (dial or digital) rather than relying on the visual appearance of the tyre, which can look fine well below the safe threshold.
• Check monthly and before any long road trip. Many vehicles manufactured after 2008 have a TPMS (tyre pressure monitoring system) that triggers a dashboard warning — but TPMS typically alerts only when pressure is already 25% or more below the recommended level. See our guide to dashboard warning lights to understand what that indicator actually means.

Tread Depth: The Legal Minimum and the Safer Benchmark

Tread depth is measured in 32nds of an inch in the U.S. New passenger tyres typically start at 10/32" to 11/32". The legal minimum in most U.S. states is 2/32" — at which point tyres must be replaced. However, independent safety research consistently shows that wet-road stopping distances increase significantly before you reach the legal minimum.

Practical Depth Thresholds to Know

The Quarter Test

Insert a quarter into the tread groove with Washington's head pointing down. If you can see the top of his head, you're at approximately 4/32" — start planning a replacement. The penny test (Lincoln's head visible = 2/32") only confirms you've hit the legal minimum, not that you're safe.

Tread wear is also a diagnostic tool. Wear only in the center points to chronic overinflation; wear on both outer edges suggests chronic underinflation; one-sided wear often signals an alignment or suspension problem. Keeping an eye on wear patterns connects directly to other routine checks that are easy to overlook, like tyre rotation, which should typically happen every 5,000–8,000 miles.

This article is for general informational purposes. Always follow your vehicle manufacturer's specifications and consult a qualified tyre professional if you have concerns about your specific tyres or driving conditions.