The string of numbers and letters on a tyre’s sidewall looks like a code, and it is — a precise, standardised description of the tyre’s size, construction and ratings. Learning to read it lets you buy the right replacements, understand why a tyre rides or handles the way it does, and judge whether a different size will fit. It also connects directly to how your car drives, because tyre size quietly changes your gearing and speedometer. Here is the whole code, decoded.
Compare sizes and check the effect of a change with the Tire Size Calculator, and confirm wheel fitment with the Bolt Pattern Reference.
Decoding 225/45R17 91V
Take a typical sidewall marking and break it apart:
| Part | Means |
|---|---|
| 225 | Tread width in millimetres, across the tyre |
| 45 | Aspect ratio — sidewall height is 45% of the width |
| R | Radial construction (almost all modern tyres) |
| 17 | Wheel diameter in inches the tyre mounts on |
| 91 | Load index — a code for maximum carrying weight |
| V | Speed rating — maximum certified speed |
Notice the units mix: width in millimetres, wheel diameter in inches, and the sidewall height given as a percentage rather than a measurement. That percentage is the key to a lot of the behaviour.
Width and aspect ratio: ride and handling
The first two numbers shape how a tyre feels. A wider tread (the 225) puts more rubber on the road for grip, at the cost of more weight, rolling resistance and tramlining. The aspect ratio (the 45) sets the sidewall height as a fraction of that width, and it is a comfort-versus-handling dial:
- Low aspect ratio (e.g. 35) — a short, stiff sidewall. Sharp, responsive handling and a planted feel, but it transmits more bumps and is easier to damage on potholes.
- High aspect ratio (e.g. 65) — a tall sidewall that flexes and cushions, giving a softer ride and more protection, but vaguer steering.
Because the height is a percentage of the width, changing either number changes the actual sidewall height — which changes the overall diameter, and that has knock-on effects.
Construction and wheel diameter
The R denotes radial construction, where the body plies run radially across the tyre; it has been the standard for decades (you may rarely see D for older bias-ply). The final number is the wheel diameter in inches — the tyre must match the wheel exactly. This is the one number you cannot fudge: a 17-inch tyre only fits a 17-inch wheel.
Load index and speed rating
After the size come two safety ratings you should never ignore. The load index is a coded maximum weight each tyre can safely carry — a higher number means more capacity. The speed rating letter is the maximum sustained speed the tyre is built for (for example T ≈ 118 mph, H ≈ 130, V ≈ 149, W ≈ 168).
Changing size: gearing and the speedometer
The crucial practical point: tyre overall diameter affects how your car drives. A bigger diameter means the wheel travels farther per revolution, which acts exactly like taller gearing — it lowers engine RPM at a given speed (quieter, more economical cruising) but reduces torque at the contact patch, making acceleration feel lazier. It also makes the speedometer read low, since the car assumes the original rolling distance per turn.
That is why tyre changes are done as a “plus size”: go to a larger wheel and a lower-profile tyre that keeps the overall diameter nearly the same (within about 3%). The handling sharpens from the shorter sidewall, but the gearing, speedometer and clearances stay correct. The Tire Size Calculator shows the diameter difference and speedometer error for any size change before you buy.
In practice
The sidewall code is a complete spec: width, sidewall proportion, construction, wheel size, and the load and speed limits. Read it to buy correctly, choose the aspect ratio for the ride-versus-handling balance you want, and keep the overall diameter close when changing sizes so your gearing and speedometer stay honest. Check any change with the Tire Size Calculator, and see how diameter ties into gear ratios.
Frequently asked questions
What does a tyre size like 225/45R17 mean?
225 is the tread width in millimetres; 45 is the aspect ratio (sidewall height is 45% of the width); R means radial construction; and 17 is the wheel diameter in inches the tyre fits. So it is a 225 mm-wide radial tyre for a 17-inch wheel with a sidewall about 101 mm tall.
What is the aspect ratio on a tyre?
It is the sidewall height as a percentage of the tread width. A lower number (like 35) means a shorter, stiffer sidewall — sharper handling but a harsher ride; a higher number (like 65) means a taller sidewall — more comfort and bump absorption. It is the middle number in the size.
Can I fit a different tyre size?
Yes, if you keep the overall diameter close to original (a "plus size"), so the speedometer stays accurate and the tyre clears the bodywork. You can change width and aspect ratio as long as the rolling diameter is within roughly 3%. Always confirm load rating and wheel fitment too.
What do the load index and speed rating mean?
The number after the size is the load index — a code for the maximum weight the tyre can carry — and the letter is the speed rating, the maximum sustained speed it is certified for. Never fit tyres rated below your vehicle’s requirement; it is a safety limit, not a suggestion.