There are two numbers on a tyre and most people only use one of them. Tread depth tells you how much of the tyre has been worn away. The four digits at the end of the DOT code tell you when the rubber was made. They are different measurements, they fail in different ways, and here the second one usually gets there first.
How do I find out how old my tyre is?
Read the last four digits of the DOT code moulded into the sidewall. That is the manufacturing date, and the format is set by regulation. The US rule, 49 CFR 574.5, requires every new tyre to carry a tire identification number of thirteen symbols, of which “the date code, consisting of four numerical symbols, is the final group.” It then says exactly what those four mean: “The first and second symbols of the date code must identify the week of the year by using ‘01’ for the first full calendar week in each year,” and “The third and fourth symbols of the date code must identify the last two digits of the year of manufacture.” For completeness, the same section defines the week: “The calendar week runs from Sunday through the following Saturday.”
So “2223” is the twenty-second week of 2023 — the end of May that year. It takes ten seconds to read and it is the single most useful thing on the tyre, because unlike everything else about a tyre it cannot be improved by a bucket of tyre shine.
Does a tyre expire?
There is no single global rule, and anybody who tells you there is has not looked. What exists is a spread of positions, and NHTSA lays it out in one sentence in its own summary of its tire aging research: “Some vehicle manufacturers recommend that tires be replaced every six years regardless of use and a number of tire manufacturers recommend 10 years.” Six from the people who built the car, ten from the people who built the tyre.
Locally, one government document we can actually read prints a figure. The RTA’s vehicle inspection brochure, in the section headed “The tyres”, instructs the driver to “Check the condition and expiry date of the tyres” and states: “Tyres expire 5 years from the date of manufacture.” That is the RTA’s published inspection guidance. We are deliberately not going to tell you it is a federal law or a traffic offence, because we could not read a document that says so — but if you are presenting a car for its annual test in Dubai, that is the number the test material itself uses.
Tread is a record of how far you have driven. The date code is a record of how long the chemistry has been running. Only one of them can be read at a glance, and it is not the one people look at.
Why does heat matter more here than mileage?
Because the ageing is chemical, and heat is what drives it. NHTSA names both mechanisms in its own summary: “The aging mechanisms that are most likely to affect the relative safety of a tire are chemical and mechanical in nature. Chemical aging occurs due to the combined effect of heat and oxygen on the rubber compound and mechanical aging results from the stresses and strains that a tire incurs during its normal use.” Mileage buys you the second one. Sitting in a car park in August buys you the first.
The agency then says where it found the failures: “Tire aging failures tend to appear in the high heat states, in the summer months, during the day, while the vehicle is being driven at highway speeds. All of these factors make it more difficult for the tire to dissipate heat, and heat build-up is the main factor in these types of failures.” Their accelerated laboratory ageing test is calibrated to a specific place: “Our current oven-aging protocol approximates the aging experienced by a tire with four years of service in Phoenix, Arizona.” Four years in Phoenix is the benchmark for severe. Nobody has published one for Sharjah.
Can a tyre look perfect and be finished?
Yes, and this is the sentence worth carrying away from the whole article. NHTSA: “Tire aging takes place whether a tire is driven or not and for this reason is a concern for spare tires and tires that are not regularly driven. The effect of aging may not be visibly detectable on these tires and their integrity may be compromised even though they could be showing a great deal of remaining tread.”
Three practical consequences follow from that one paragraph. The spare in the boot is ageing on the same clock as the four on the road, and it is the one tyre nobody looks at until the worst possible moment. A low-mileage car — the second car, the weekend car, the one that sat while somebody was abroad — has old tyres with new-looking tread. And a nearly-new set that has been stacked in a warehouse for three years before it was fitted has three years already spent.
What pressure should I run in this heat?
The pressure on the placard, measured cold. The number on the door pillar or in the handbook is the vehicle manufacturer’s figure for your car; the number moulded into the tyre sidewall is the tyre’s maximum, not a recommendation. The US Tire Manufacturers Association is specific about where to look: “You’ll find recommended pressure on a label on the driver’s door or in your vehicle owner’s manual.”
“Cold” is not a figure of speech, and here it is the hard part. USTMA defines it as tyres that have been “parked for at least three hours”, and gives the rate of change: inflation pressure “increases (in warm weather) or decreases (in cold weather) one to two pounds for every 10 degrees of temperature change.” Check a hot tyre at a petrol station on a July afternoon and it will read high — and if you bleed it down to the placard figure, you have just set the car several psi under-inflated for every cold start after it.
Isn’t that what the TPMS light is for?
No. The tyre pressure monitoring lamp is a late warning, not a gauge, and the regulation that created it says how late. FMVSS No. 138 requires the system to “Illuminate a low tire pressure warning telltale not more than 20 minutes after the inflation pressure in one or more of the vehicle’s tires … is equal to or less than either the pressure 25 percent below the vehicle manufacturer’s recommended cold inflation pressure”. Twenty-five per cent below placard is a long way below placard. A car can be meaningfully under-inflated — wearing its shoulders, running hot, using fuel — for months with no lamp at all.
Why does under-inflation matter so much in heat?
Because the two problems are the same problem. A tyre that is soft flexes more, and flexing is how a tyre makes heat; a tyre that is already hot has less margin to absorb it. NHTSA connects the two directly, noting that mandatory tyre pressure monitoring “has helped alert consumers to under-inflation that is also known to degrade tires faster”, and describing the conditions under which ageing failures actually appear: high heat, summer, daytime, highway speeds — “All of these factors make it more difficult for the tire to dissipate heat, and heat build-up is the main factor in these types of failures.”
Which is why the summer habit of letting air out of a hot tyre is the exact wrong move. The pressure is high because the tyre is hot, not because it is over-inflated; bleed it and you have set the car up to run soft — and therefore hotter — on every journey afterwards. If you have to check pressures in the middle of the day, check them, note that they will read above the placard, and set them properly the next morning before the car has moved.
How much tread is enough?
USTMA’s threshold is plain: “Once your tread wears down to 2/32nds of an inch, it’s time to visit your local tire shop.” That is about 1.6 mm, and it is a minimum rather than a target — wet grip falls away well before a tyre reaches it. We should say clearly that the RTA inspection brochure we read does not print a tread-depth figure at all; it asks the driver to “Inspect the validity of the tyres and ensure they are free from defects” and to “Verify the appropriate tyre pressure”. So the depth number above is the tyre industry’s, not a local legal minimum we can show you.
What people check, and what it actually tells them
| What is checked | What it tells you | What it does not tell you |
|---|---|---|
| Tread depth | How much of the tyre has been worn off by use. | Anything at all about the age or condition of the rubber underneath. |
| The DOT date code | The week and year the tyre was manufactured. | How it was stored, or how many summers it has actually stood through. |
| The TPMS lamp | That at least one tyre is 25% or more below placard pressure. | That the other three are correct. It is a floor, not a gauge. |
| A pressure reading at a petrol station in the afternoon | The hot pressure of that tyre at that moment. | The cold pressure, which is the only one the placard figure refers to. |
| A visual check from the kerb | Obvious damage, an obviously flat tyre, tread across the centre. | Inner-shoulder wear, sidewall condition on the inside face, or age. |
The short version
- Read the last four digits on all five tyres, including the spare. It costs nothing.
- Set pressure cold, to the placard, not to the sidewall maximum, and not after a drive.
- Do not treat the TPMS lamp as a pressure check. It is a 25-per-cent-low alarm.
- Judge age and tread separately. A tyre can pass one and fail the other.
- Have the inner shoulders looked at when the wheels are off — that is where an alignment fault shows up first, and it is not visible with the wheel on.