A tyre sidewall code like 225/45R17 94V packs in the width in millimeters (225), the aspect ratio as a percentage of that width (45), the construction type (R for radial), the wheel diameter in inches (17), the load index (94), and the speed rating (V) — and a separate four-digit DOT code tells you the exact week and year the tyre was manufactured. That last piece, tyre age, is the one most drivers never learn to check, even though rubber degrades with age regardless of how much tread remains.
Breaking Down the Size Code
| Example: 225/45R17 94V | Meaning |
|---|---|
| 225 | Section width in millimeters, measured sidewall to sidewall |
| 45 | Aspect ratio — sidewall height as a percentage of the width (here, 45% of 225mm) |
| R | Radial construction — standard on virtually all modern passenger tyres |
| 17 | Wheel diameter the tyre is designed to fit, in inches |
| 94 | Load index — a code number corresponding to the maximum weight the tyre can carry |
| V | Speed rating — the maximum speed the tyre is rated to sustain safely |
Load Index: What the Number Actually Means
The load index is a coded number, not a direct weight — a lookup table converts it to an actual maximum load per tyre. A load index of 94, for example, corresponds to roughly 670 kg (about 1,477 lbs) per tyre. Fitting tyres with a lower load index than your vehicle’s original specification is not a minor detail — it means the tyre may be rated below what your fully loaded vehicle can actually put on it.
Speed Rating: What the Letter Means
| Letter | Maximum speed |
|---|---|
| S | 112 mph (180 km/h) |
| T | 118 mph (190 km/h) |
| H | 130 mph (210 km/h) |
| V | 149 mph (240 km/h) |
| W | 168 mph (270 km/h) |
This is a sustained-speed capability rating, not a recommendation to drive at that speed — but fitting a tyre with a rating below your vehicle manufacturer’s specification is still worth avoiding, since it can affect handling characteristics the vehicle was tuned around even well below the rated maximum.
The DOT Code: Reading a Tyre’s Actual Age
Look for a code starting with “DOT” followed by a string of characters ending in four digits — those last four digits are what matter for age. The first two digits are the week of manufacture, and the last two are the year. A tyre marked 2223 at the end was made in week 22 of 2023.
Rubber compounds degrade chemically over time from heat, UV exposure, and ozone, regardless of how many miles have been driven. Many tyre manufacturers recommend replacement around 6-10 years from the manufacture date even with adequate tread remaining — a genuinely common surprise for owners of low-mileage vehicles or a rarely-used spare tyre that looks fine but may be older than it appears.
How to Check This on Your Own Car
- Find the size code on the sidewall and compare it against your vehicle’s specification, usually printed on a sticker inside the driver’s door jamb or in the owner’s manual.
- Locate the DOT code and check the last four digits for manufacture date, particularly on a spare tyre or any tyre that’s been in service for several years.
- Confirm all four tyres match in size, load index, and speed rating unless your vehicle specifically uses a staggered fitment — mismatched tyres can affect handling balance.
Never work under a vehicle supported only by a jack. Use axle stands on firm, level ground, chock the wheels that stay down, and leave the car in gear or Park with the handbrake on. A jack lifts; stands hold.
When to See a Mechanic or Tyre Shop
Worth a visit if you’re unsure whether a replacement tyre matches your vehicle’s original specification, or if a DOT code check reveals a tyre older than 6-10 years regardless of tread depth — including a spare that’s never been used but has aged in the trunk. More in our maintenance section.
Frequently Asked Questions
What do the numbers on a tyre sidewall mean?
They encode width, aspect ratio, construction type, wheel diameter, load index, and speed rating — for example, 225/45R17 94V breaks down into 225mm width, 45% aspect ratio, radial construction, a 17-inch wheel, load index 94, and speed rating V.
How do I tell how old a tyre is?
Look for the DOT code on the sidewall — the last four digits give the week and year of manufacture. A code ending in 2223 means week 22 of 2023.
Does tyre age matter if there’s still plenty of tread?
Yes — rubber degrades chemically from heat, UV, and ozone exposure over time regardless of tread depth. Many manufacturers recommend replacement around 6-10 years from manufacture even with tread remaining.
What is a tyre’s load index?
A coded number that corresponds to the maximum weight the tyre can safely carry, found via a lookup table — it’s not the weight itself, just a reference code.
Can I fit a tyre with a lower speed rating than my car’s original spec?
It’s best avoided — the vehicle was tuned around the original rating’s handling characteristics, and a lower-rated tyre may not perform the same even at speeds below its rated maximum.
