A tire sidewall tells you more than its size. Start with the size code, then read the load and speed information, the manufacturing date, and the markings that affect installation and use. Here is how to work through them using 305/35ZR20, the example in Vicrez’s short video.
Watch: What Does 305/35ZR20 Mean?
This 28-second Vicrez video walks through the size code. The explanation below expands on the same example and adds the other markings to check before ordering.
Watch the original YouTube Short. The video’s spoken size explanation is also covered in the readable table below.
Read the Size Code One Part at a Time
| Marking | What it tells you | What to remember |
|---|---|---|
| 305 | Nominal section width: 305 millimeters, measured sidewall to sidewall under specified conditions. | It is not the tread-contact width. Actual dimensions depend on the tire and measuring rim. |
| 35 | Aspect ratio: nominal sidewall height is 35% of nominal section width. | It is a percentage, not 35 millimeters. |
| Z | A high-speed designation within the size marking. | Read the tire’s separate speed symbol and manufacturer information; Z alone does not supply the full service description. |
| R | Radial construction. | R describes construction here. It is not the wheel radius. |
| 20 | Nominal wheel bead-seat diameter: 20 inches. | It is not the tire’s overall outside diameter or the wheel’s width. |
Keep the full size together when searching. 305/35ZR20 and 305/30ZR20 share the same first and last numbers but specify different aspect ratios. Similar-looking size names do not make tires interchangeable.
Why 35 Means About 107 mm in This Example
Multiply nominal width by the aspect ratio expressed as a decimal:
Rounded to the nearest millimeter, that is about 107 mm of nominal sidewall height.
This is a calculation from the size code, not a measurement of an installed tire. It does not account for the full details of the tire’s construction, approved measuring rim, inflation or loading. Use the manufacturer’s dimensional data when clearance or vehicle-system compatibility matters.
The same 35 aspect ratio produces a different nominal sidewall height on a different width: 275 × 0.35 = 96.25 mm. That is why comparing the middle number alone can be misleading.
Read Load Index, Speed Symbol and XL Separately
Look near the size code for the service description: typically a numerical load index followed by a speed symbol. Record these along with the complete size, rather than assuming every tire in that size has the same ratings.
- Load index identifies load capacity under specified conditions. Have the tire professional confirm capacity and inflation requirements for the vehicle and axle.
- Speed symbol describes a tire capability under defined test conditions. It is not a recommendation to drive at that speed or a guarantee about an aged, damaged or improperly inflated tire.
- XL / Extra Load is a load designation. Read it with the load index and applicable inflation information; the letters alone are not a complete load specification.
For a particular VCORSA SKU, check the current technical information and the actual tire supplied. This guide explains the notation without assigning an unverified load index, speed symbol or load designation to the entire range.
Find the DOT Tire Identification Number and Date Code
The Tire Identification Number, or TIN, includes manufacturing identification information. Its final four digits identify the week and year of manufacture. In an illustrative date code of 1425, 14 is the week and 25 is the year 2025. It is not a size or a warranty-expiration date.
The complete TIN and date code may require checking the other sidewall. Ask a tire professional for help if it cannot be inspected safely. Keep the full code for registration and recall checks; the four date digits alone do not identify every relevant manufacturing detail. The date-code format is explained in NHTSA’s tire-identification requirements.
A recent date code does not replace an inspection. Follow the tire and vehicle makers’ service-life recommendations and have cuts, bulges, cracking, impacts or unexplained pressure loss assessed.
Other Sidewall Markings Worth Reading
Maximum Pressure Versus the Vehicle’s Recommended Pressure
The maximum pressure marked on a tire is not automatically the pressure to use on your car. Start with the vehicle’s Tire and Loading Information label and owner’s manual. For a changed tire or wheel setup, have the appropriate inflation requirements confirmed. NHTSA TireWise explains placard pressure and checking tires when cold.
UTQG: Three Different Grades
Where applicable, Uniform Tire Quality Grading separates treadwear, traction and temperature. They describe different aspects of the grading system. Treadwear is not a mileage warranty; traction grading is not a complete road test of cornering or every wet-weather condition. Read the complete grades for the actual tire instead of treating a lone number as a full performance score.
Direction, OUTSIDE and Seasonal Markings
A rotation arrow or an OUTSIDE marking gives installation information when present. Do not assume every tread uses the same orientation. Similarly, confirm the tire’s intended seasonal use from its actual markings and manufacturer information. A size code or a performance label alone does not establish severe-snow suitability or run-flat capability.
Turn the Markings Into a Better Tire Question
Photograph the complete size and service description, check the vehicle’s label and manual, and record your wheel size. If the wheels, suspension or bodywork have changed, include those details and the front/rear axle. The tire already on a modified vehicle is a useful record, but its presence does not prove the setup is correct.
For example, VCORSA VT101 is listed in 305/35ZR20. That catalog identity is a starting point for shopping; wheel-width approval, load and speed requirements, clearance and vehicle compatibility still need to match the intended setup.
Use the live VCORSA catalog for current sizes and purchasing information, then use the buying checklist to confirm the complete order.
Browse VCORSA sizesRead the buying checklistOnce you have recorded the tire identity, use our directional and asymmetric mounting-markings guide to prepare the installer questions. Keep the full identification photographs with your tire registration and recall record.
For the capacity decision behind the markings, continue with load index and XL versus standard load. It explains what to confirm when a listing, sidewall and proposed pressure need to be reconciled.
Common Tire-Sidewall Questions
Does a larger first number guarantee more grip?
No. Width alone does not establish the contact patch, grip or braking performance. Tire design, load, inflation, wheel width, vehicle setup and conditions all matter.
Does 20 mean the tire is 20 inches tall?
No. It is the nominal wheel bead-seat diameter. The tire’s outside diameter also includes the tire section above the wheel.
Can the size code tell me which wheel width to buy?
It tells you wheel diameter, but it does not give the full approved wheel-width range. Check the exact tire’s technical data and the wheel’s specifications.
Where should I start if the markings are hard to read?
Check the vehicle’s label and manual, then ask a tire professional to inspect the tire and confirm its identity. Do not guess a partially visible size or rating when ordering.
Sources and Visual Notes
This guide uses the supplied Vicrez video, NHTSA TireWise, NHTSA tire-identification requirements, and the USTMA Care and Service manual for terminology. The tire photograph is a supplied Vicrez promotional asset. The infographic is an AI-generated educational illustration, with its numerical explanation also provided in accessible text.
