On the tire sidewall, you'll find a combination of numbers and letters like 225/45 R17 91V – and this tire designation contains all the important information: width, height, diameter, load capacity, and permissible speed. If you can read it, you'll find the right tires and rims faster and avoid expensive mispurchases. In this guide, we'll decipher each specification – in detail and with practical relevance.
1. The Tire Designation at a Glance
Let's take the example 225/45 R17 91V. Each specification has a fixed meaning:
| Specification | Meaning |
|---|---|
| 225 | Tire width in millimeters |
| 45 | Aspect ratio (Height = 45% of width) |
| R | Construction type: Radial tire |
| 17 | Rim diameter in inches |
| 91 | Load index (load capacity) |
| V | Speed index |
2. Why the Correct Tire Size is Important
Tire size is more than a technical specification – it affects safety, handling, and legality. An incorrect size can cause the tire to rub in the wheel well, the speedometer to display incorrectly, or insufficient load capacity. In the worst case, the operating permit expires, and insurance coverage is jeopardized. Therefore, only use sizes approved for your vehicle – either according to your vehicle registration certificate or the rim's ABE (General Operating Permit). If you can read the designation, you'll immediately recognize if a tire fits and avoid mispurchases.
3. Tire Width (225)
The first number indicates the nominal width of the tire in millimeters – measured from sidewall to sidewall. In our example, this is 225 mm. Wider tires tend to offer more grip on dry roads but consume slightly more fuel and can hydroplane sooner in wet conditions. The width must match the rim width – the guide Rim Width and Tire Width explains how this relates.
4. Aspect Ratio (45)
The second number is the aspect ratio (also height-to-width ratio) in percent. A 45 means: The height of the tire sidewall is 45% of the width – i.e., 45% of 225 mm, approximately 101 mm. The lower the number, the flatter the tire. Low-profile tires look sporty and improve steering precision but offer less comfort and are more sensitive to potholes.
5. Construction Type (R)
The letter stands for the construction method. R means radial tire – the common construction type today, where the carcass plies run radially (perpendicular to the direction of travel). Rarely, you might still find D (diagonal tire), for example, on classic cars or special trailers. A ZR indicates particularly high-speed approvals.
6. Rim Diameter (17)
The number after the construction type letter is the rim diameter in inches – here, 17 inches. The tire and rim must precisely match in inch diameter: a 17-inch tire belongs on a 17-inch rim. If you upgrade to a larger rim, you will need a tire with a smaller aspect ratio to keep the rolling circumference the same.
Convert Inches to Centimeters
One inch is exactly 2.54 cm. To convert, multiply the inch value by 2.54:
- 16 inches = 40.6 cm
- 17 inches = 43.2 cm
- 18 inches = 45.7 cm
- 19 inches = 48.3 cm
- 20 inches = 50.8 cm
Important: These values refer to the rim diameter, not the overall diameter of the wheel – the latter is significantly larger due to the tire.
7. Load Index (91)
The load index (also load capacity rating) indicates how much weight a single tire can carry. The 91 stands for 615 kg per tire. This value must not fall below the load index required in the vehicle documents. A complete conversion table and details can be found in the guide Load Index for Tires.
8. Speed Index (V)
The last letter is the speed index – the maximum permissible speed of the tire. The most common values:
| Index | Maximum Speed |
|---|---|
| T | 190 km/h |
| H | 210 km/h |
| V | 240 km/h |
| W | 270 km/h |
| Y | 300 km/h |
The speed index of the tire must not fall below the value required in the vehicle documents. An exception is winter tires: a lower index is permissible here if a corresponding notice is placed within the driver's field of vision.
9. Additional Markings on the Tire
In addition to the basic size, many tires carry other markings:
- M+S: Mud and Snow – since October 2024, this marking alone is no longer sufficient for winter tires.
- Alpine Symbol (3PMSF): Snowflake in the mountain – the genuine winter tire marking. Details in the guide AT Tires in Winter.
- XL / Reinforced: reinforced version with a higher load index.
- C: Commercial – van tires, often with a double load index.
- RunFlat (RF, RSC, ROF, etc.): run-flat tires that can be driven for a limited distance after pressure loss.
- DOT Number: date of manufacture (week/year) – important for tire age (see Tread Depth & Tire Age).
The EU Tire Label
Regardless of the size designation, every new tire carries an EU tire label. It evaluates three characteristics and helps with comparison:
- Fuel efficiency: rolling resistance and thus impact on consumption (scale A to E).
- Wet grip: braking performance on wet roads (A to E) – a safety-relevant value.
- External rolling noise: pass-by noise in decibels.
The label does not replace the correct size but is a useful criterion when choosing between several suitable tires.
10. Where Can I Find My Tire Size?
There are three reliable sources:
- On the tire sidewall of your current tires.
- In your vehicle registration certificate (registration document Part I), field 15 – it lists the factory-approved sizes. More on this in the guide Vehicle Registration Certificate Explained.
- In the door jamb or fuel filler flap, sometimes together with the recommended tire pressure.
11. Can I Use a Different Tire Size?
Basically yes – but only if the size is approved for your vehicle. Permitted are the sizes listed in the vehicle registration certificate, as well as other combinations approved by the ABE (General Operating Permit) or the parts certificate of the rim. It is important that the rolling circumference remains as consistent as possible so that the speedometer and vehicle electronics function correctly. If you change the width, you typically adjust the aspect ratio to maintain the overall diameter.
Plus-Sizing: Larger Rim, Flatter Tire
If you upgrade to a larger rim ("plus-sizing"), you compensate for the larger inch diameter with a lower tire aspect ratio. An example: Instead of 205/55 R16, you use 225/45 R17. The rim becomes 1 inch larger, the tire wider and flatter – but the overall diameter remains almost the same. This maintains the correct speedometer reading and clearance in the wheel well. Visually, the larger wheel fills the wheel well better, and the flatter tire looks sportier – at the expense of some comfort. The crucial point remains: the specific combination must be approved for your vehicle.
Millimeter Specifications and Rolling Circumference
Small deviations in rolling circumference of a few percent are usually uncritical and often covered by the approval. Larger deviations, however, lead to incorrect speed display and can impair assistance systems. When in doubt, stick to the combinations listed in the vehicle registration certificate or expert opinion – then the rolling circumference and approval will automatically be correct.
12. Tire Size and Rim in zrshop
Once you know your tire size, you can quickly find suitable tires and pre-mounted complete wheels in the zrshop. Make sure that the size, load index, and speed index match your vehicle and the rim – and for all-season use, pay attention to the Alpine symbol.
Discover tires in the zrshop now →
FAQ: Frequent Questions About Tire Size
What does 225/45 R17 91V mean?
225 is the tire width in mm, 45 is the aspect ratio (height = 45% of width), R is the radial construction, 17 is the rim diameter in inches, 91 is the load index (615 kg), and V is the speed index (240 km/h).
Where can I find my tire size?
On the sidewall of your current tires, in the vehicle registration certificate (registration document Part I, field 15), and often in the door jamb or fuel filler flap. The vehicle registration certificate lists the factory-approved sizes.
What does the aspect ratio mean?
It indicates the height of the tire sidewall as a percentage of the width. For 225/45, the sidewall is 45% of 225 mm high, which is about 101 mm. The smaller the number, the flatter and sportier the tire.
Can I use a different tire size than what is in the registration certificate?
Yes, if the size is approved for your vehicle – either in the vehicle registration certificate or via the ABE (General Operating Permit) or the expert opinion for the rim. The rolling circumference should remain as consistent as possible.
What does the speed index mean?
It indicates the maximum permissible speed of the tire: T = 190, H = 210, V = 240, W = 270, Y = 300 km/h. The index must not fall below the value required in the vehicle documents.
What do XL or Reinforced mean on the tire?
XL (Extra Load) and Reinforced indicate a reinforced version with a higher load index for the same size – useful for heavy vehicles such as SUVs, vans, and motorhomes.
What does R in the tire size mean?
The R stands for radial construction – the common construction type today. Rarely, there is a D for diagonal tires (e.g., for classic cars) or ZR as an indication of particularly high-speed approvals.
Can I use wider tires?
Only if the wider combination is approved for your vehicle – via the vehicle registration certificate or the ABE of the rim. Wider tires often require a larger rim and a lower aspect ratio to ensure the rolling circumference is correct and nothing rubs.
Do the front and rear tire sizes have to be the same?
For most vehicles, yes. However, some sporty vehicles come from the factory with staggered tire sizes, featuring wider tires on the rear axle. The decisive factor is always the approval for your vehicle.
Sources
- Standardized tire marking (ETRTO) – width, aspect ratio, construction type, load and speed index
- Marking of winter tires – Alpine symbol (3PMSF), M+S
- Vehicle registration certificate Part I – Field 15 (permitted tires)