Tyre Basics Every Student Driver Should Understand

Reading the Sidewall Code

Every tyre has a string of letters and numbers moulded into its sidewall. It looks complex at first, but each segment has a specific meaning. Take the example code 205/55R16 91V:

• 205 — Tyre width in millimeters
• 55 — Aspect ratio: the sidewall height is 55% of the tyre's width
• R — Radial construction (standard on most passenger vehicles)
• 16 — Rim diameter in inches
• 91 — Load index (the maximum weight each tyre can carry)
• V — Speed rating (the maximum safe sustained speed)

You don't need to memorize what every load index or speed rating number means. What matters is that your replacement tyres match the specification listed on your door jamb sticker — using a mismatched size or rating can affect handling and safety.

The sidewall also contains a DOT code ending in a four-digit number. This is the manufacture date: the first two digits are the week and the last two are the year. If your tyres show "3521," they were made in the 35th week of 2021. This matters because rubber degrades with age, even when tread looks fine. See the FAQ below for guidance on tyre age limits.

For a broader look at how your car's systems connect, check out our plain-language guide to how cars work.

Tyre Pressure: The Check You Should Do Every Month

Tyre pressure is measured in PSI. Driving on underinflated tyres increases the contact area between the tyre and the road, generating excess heat — one of the primary causes of blowouts. Overinflated tyres, on the other hand, reduce grip and wear unevenly down the centre of the tread.

To check pressure correctly:

1. Check when tyres are cold — before driving or after less than one mile.
2. Find the recommended PSI on the sticker inside your driver's door jamb.
3. Remove the valve cap from each tyre's stem.
4. Press a tyre pressure gauge firmly onto the valve stem and read the result.
5. Add air if needed using an air compressor (available at most gas stations).
6. Replace valve caps securely when done.

Cold weather is a common culprit for pressure drops. For every 10°F the temperature falls, tyre pressure decreases by roughly 1 PSI. Students in northern states should check pressure more frequently during fall and winter. Tyre pressure habits also connect directly to mistakes new drivers frequently make with car care.

Tread Depth and When to Replace

Tread channels water away from the contact patch between the tyre and the road. When tread wears down, a wet road becomes significantly more dangerous because water has nowhere to go — the result is hydroplaning, where the tyre skims across water rather than gripping the surface beneath it.

In most U.S. states, the legal minimum tread depth is 2/32 of an inch. Safety experts generally recommend replacing tyres before they reach this point. The easiest way to check:

• Penny test: Insert a penny with Lincoln's head down into a tread groove. If the top of his head is visible, the tread is dangerously low.
• Quarter test: Use a quarter the same way. Seeing the top of Washington's head means you're at about 4/32 — still legal, but time to plan for replacement soon.
• Tread wear indicators: Most tyres have small raised bars moulded into the tread grooves. When the tyre surface becomes flush with these bars, you're at the legal minimum.

Check multiple spots across the width of each tyre — uneven wear can signal alignment or inflation problems that a mechanic should assess. Good tyre condition is especially important in wet conditions. Driving in bad weather near campus covers how worn tyres increase risk in rain and ice.

Make tyre checks part of a regular habit alongside your other vehicle checks — the pre-journey checklist for first-time owners is a useful starting point.

This article is for general informational purposes. For tyre replacement, installation, or any concerns about your vehicle's safety, consult a qualified mechanic.