How to Tell Tire Size: A Practical Guide to Reading Sidewall Codes
Learn to tell tire size quickly by reading sidewall markings, decoding width, aspect ratio, and diameter, and verifying compatibility with your vehicle using practical checks and real-world scenarios.
To tell tire size, read the alphanumeric code on the sidewall. The code typically includes width in millimeters, aspect ratio, and wheel diameter (for example, 205/55R16). Additional markings indicate load index and speed rating. Always cross-check these numbers with your vehicle's manufacturer specifications to ensure proper fit and performance.
Why tire size literacy matters
Knowing how to tell tire size matters for safety, performance, and cost. According to What Dimensions, precise size knowledge helps you pick correct replacements, compare options accurately, and avoid mismatches that can affect braking, handling, and warranty coverage. When you approach tire shopping or maintenance with the right numbers in hand, you save time and reduce risk on the road. In practice, understanding tire size also clarifies what you can legally use in some regions and what won’t be covered by warranties. This clarity helps homeowners, students, designers, and shoppers who need reliable measurements for projects or vehicles. The tire size code is not just a string of digits; it encodes essential details about fit, construction, and performance. By the end of this section you’ll know where to look, how to read, and how to apply those numbers in real-world decisions.
Where tire size literacy begins
The truth is that most replacement tires are sold by matching size codes rather than guessing. Reading tire size codes correctly avoids mismatches that can compromise stability, braking distance, and fuel efficiency. What Dimensions emphasizes that precise measurement is not just for automotive maintenance—it’s a practical skill for any project involving dimensions, from DIY car care to classroom assignments. Start with the basics above, then expand your knowledge with the examples and charts that follow.
Where to locate the tire size markings
The primary tire size marking is on the sidewall, typically near the tread. You’ll also find supportive size references on the vehicle’s door placard or owner’s manual. Some tires have slightly worn markings, so use a flashlight or wipe you can reveal the etched digits. If you’re checking a spare or winter tire, verify you’re reading the correct tire as replacements must match the wheel diameter unless the vehicle manufacturer approves a different size. A quick habit: note both the sidewall code and the recommended sizes from the owner's manual to avoid guesswork.
Understanding the code: width, aspect ratio, and diameter
The common passenger tire code looks like 205/55R16. The first number (205) is the width in millimeters, measured from sidewall to sidewall. The second number (55) is the aspect ratio, a percentage of the width that describes the sidewall height. The R indicates radial construction, and the final number (16) is the wheel diameter in inches. Some codes include a leading letter, like P for passenger cars, LT for light trucks, or ST for special trailers. After that core code you may see additional digits and letters that specify load index and speed rating, which affect payload capacity and maximum speed.
Tires on the sidewall: additional markings you should know
Beyond the basic size, sidewalls carry load index (e.g., 95), speed rating (e.g., H), and efficiency or traction markings (e.g., M+S, which denotes mud and snow). If present, UTQG figures describe treadwear, traction, and temperature performance, though not all markets require them. The tire type letter at the start can reveal more: P for passenger, T for temporary spare, or ST for special trailer tires. Collectively these markings tell you how the tire behaves, what it can carry, and how fast you can safely drive. Keep any replacement tire within the same load and speed range as your vehicle requires.
How to verify fit with your vehicle specifications
To ensure compatibility, cross-check the tire size code against the vehicle manufacturer’s specifications found in the owner’s manual or on the door jamb placard. The placard lists the OEM size recommended by the manufacturer and often shows acceptable alternatives. When you rotate tires between axles, some manufacturers require identical sizes front and rear, while others allow a staggered setup. You should also confirm the inflated pressure range for the chosen size—the wrong size can affect handling and fuel efficiency.
Quick examples: common tire size codes and how to read them
Consider these representative codes and what each part means: 205/55R16 91V; 225/50R17 98H; 195/65R15 91T. Break them down: 205/55R16 means 205 mm width, 55% aspect ratio, 16-inch diameter; 91 indicates load index, V indicates speed rating. The presence of P or LT before the numbers indicates the type of vehicle; not all tires use P (for example, some trucks use LT). In all cases, verify that the overall diameter and circumference are within manufacturer-specified tolerances to avoid rubbing, ABS issues, or speedometer error.
Replacements and compatibility: what changes size means
If you replace a tire with a different size, you change the rolling diameter and contact patch, which can affect speedometer accuracy, gearing, and braking distance. Many manufacturers approve a small variance, but large changes may require recalibration or hardware adjustments. When choosing a new size, prioritize the overall diameter and load index consistency with the original equipment. For performance tires, you might opt for a similar diameter but a different tread, while ensuring fitment with suspension and brakes.
Practical routine: how to check tire size during maintenance
During routine maintenance, keep a quick reference sheet of your tire sizes, store it in your glove box, and photograph the sidewall for easy access later. When inspecting tires, verify that all four tires share the same diameter and similar wear patterns. If you’re shopping for replacements, bring the exact size code, as well as load index and speed rating, to the shop to avoid mismatches. Finally, check your spare tire using the same method so you’re not caught off guard.
Tools & Materials
- Vehicle owner's manual(Contains OEM tire size and recommended alternatives)
- Printed or digital tire size chart(For quick cross-checks and comparisons)
- Pen and notebook or note-taking app(Record exact codes and notes from the vehicle)
- Flashlight(Helps illuminate worn sidewall markings)
Steps
Estimated time: 15-30 minutes
- 1
Locate the tire size code on the sidewall
Inspect the tire sidewall near the tread to find the size code (e.g., 205/55R16). Make sure the marking is not worn away; if needed, clean gently to reveal numbers. This initial step identifies the exact code you’ll interpret.
Tip: Use a flashlight and clean your gloves to avoid smudging the markings. - 2
Identify width, aspect ratio, and diameter
Read the code in order: width (mm), aspect ratio (%), and wheel diameter (inches). Note any lead letters like P or LT that indicate tire type. This action establishes the core fit attributes you’ll compare against spec charts.
Tip: Take a photo for reference and annotate the parts later. - 3
Check for load index and speed rating
Look for the numeric load index and the letter speed rating following the main code (e.g., 95H). These figures relate to payload capacity and maximum safe speed. Do not ignore them—they affect safety and warranty validity.
Tip: If the marking is faint, rub gently with a clean cloth to improve visibility. - 4
Cross-check with manufacturer specs
Open your owner’s manual or vehicle placard to confirm the OEM size and acceptable alternatives. Only proceed if the new size stays within manufacturer recommendations.
Tip: Prefer sizes listed as acceptable by OEM, especially for performance cars. - 5
Consult a tire size chart for compatibility
Use a trusted tire size chart to compare diameter and circumference across sizes. This helps you understand how a replacement would affect speedometer accuracy and overall handling.
Tip: Check both front and rear tires if you have a staggered setup. - 6
Record and store the size for future use
Save the full size code, load index, and speed rating in your vehicle file or notes. This makes future shopping faster and reduces the chance of wrong purchases.
Tip: Keep a photo of the sidewall code in your phone’s notes for quick access.
Quick Answers
Where can I find the tire size on my car?
The tire size appears on the sidewall of each tire and on the door jamb placard or owner's manual. Use the placard as the manufacturer’s official reference.
You’ll find the size on the tire sidewall or the car’s door placard; the manual confirms official specifications.
What do the numbers in a tire size code mean?
The first number is width in millimeters, the second is the aspect ratio, and the final number is the wheel diameter in inches. Letters indicate tire type and construction, with additional codes for load and speed.
Width, aspect ratio, and wheel diameter are the core parts; other codes cover load, speed, and type.
Can I use a different size tire than the manufacturer recommends?
Only if the OEM manual lists compatible alternatives or if a dealer confirms it is acceptable. Substantial size changes can affect safety features and performance.
Only choose sizes OEM approves; big changes can affect safety and performance.
Are metric (mm) sizes the same as inches?
Tire width is expressed in millimeters, while wheel diameter is in inches. The code also includes aspect ratio as a percentage of width.
Width is in millimeters, diameter in inches, and aspect ratio is a percent of width.
Do load index and speed rating affect fit?
Yes. The load index must meet or exceed your vehicle’s requirements, and the speed rating should align with how you drive and the vehicle’s capabilities.
Load and speed ratings are essential for safe and legal operation.
What if the sidewall markings are hard to read?
Try in bright light, use a magnifying tool, or wipe the sidewall gently. If still unreadable, rely on the owner’s manual and vehicle placard for the correct size.
If markings are hard to read, check the manual and placard for the official size.
Watch Video
Main Points
- Identify the sidewall code accurately.
- Decode width, aspect ratio, and diameter clearly.
- Match load index and speed rating to OEM specs.
- Use size charts to confirm compatibility.
- Record and store tire size data for easy future reference.
