How to Read Tire Size:
Complete Beginner's Guide

Confused by tire sidewall markings? Learn exactly what every number, letter, and symbol means in simple terms. Written by ASE-certified technicians with 15+ years experience.

👨‍🔧
Michael Rodriguez✓ ASE Master Certified

Lead Automotive Technician

• ASE A8 - Engine Performance• ASE A4 - Suspension & Steering• 12+ Years Experience
📅Published:
🔄Updated:
✓Expert-verified content following our editorial standards
⏱️
5-Minute Read
Master tire sizing basics • Understand what to buy • Avoid costly mistakes

The Tire Size Breakdown (Visual Guide)

EXAMPLE TIRE SIZE
P 235 / 65 R 17 103 H
P
Service
Type
235
Width
(mm)
65
Aspect
Ratio %
R
Radial
Type
17
Rim
(inches)
103H
Load
& Speed
P

1. Service Type (Optional Prefix)

P = Passenger car tire

LT = Light Truck (pickups, heavy SUVs)

ST = Special Trailer tire

T = Temporary spare

(none) = Euro-metric tire

Why it matters: P-metric and Euro-metric tires have different load capacities even if other numbers match!
235

2. Section Width (millimeters)

Width of tire from sidewall to sidewall when mounted and inflated.

235mm = approximately 9.3 inches wide

Common ranges:

  • • 145-195mm = Compact/economy cars
  • • 205-225mm = Mid-size sedans/SUVs
  • • 235-255mm = Large cars/trucks
  • • 265-315mm = Trucks/performance cars
Pro tip: Wider tires = better grip but worse fuel economy and higher cost.
65

3. Aspect Ratio (Percentage)

Sidewall height as percentage of width

Example: 65 means sidewall is 65% of 235mm
= 0.65 × 235 = 152.75mm tall (6 inches)

Lower numbers (30-45) = Short sidewall = Sportier

Higher numbers (65-75) = Tall sidewall = Comfortable

Common mistake: People think this is degrees - it's NOT! It's a percentage ratio.
R

4. Construction Type

R = Radial construction (99% of modern tires)

D = Diagonal/bias-ply (vintage cars only)

B = Belted bias (extremely rare)

What's radial? Internal cords run perpendicular to direction of travel. Provides better handling, fuel economy, and tire life vs. older bias-ply design.

Modern standard: If you see "R", you're looking at standard modern tire construction.
17

5. Rim Diameter (inches)

Inside diameter of tire / diameter of wheel it fits

Critical: This is in INCHES, not millimeters!
17" tire ONLY fits 17" wheel - no exceptions!

Common sizes:

  • • 14-16" = Older/economy cars
  • • 17-18" = Most modern sedans/SUVs
  • • 19-21" = Luxury/performance cars
  • • 22-24" = Trucks, luxury SUVs
Remember: You cannot mix rim sizes! 18" tire won't fit 17" wheel.
103H

6. Load Index & Speed Rating

103 = Load Index (capacity code)

103 = 1,929 lbs max load per tire

H = Speed Rating (max safe speed)

H = 130 mph (210 km/h)

Common speed ratings:
S=112 mph • T=118 mph • H=130 mph
V=149 mph • W=168 mph • Y=186 mph

⚠️ Safety Critical: NEVER downgrade load index or speed rating below vehicle requirements!

Real-World Examples: Reading Common Tire Sizes

EXAMPLE: HONDA CIVIC
P195/65R15 89H
P
P = Passenger car tire (designed for daily commuting)
195
195mm width (7.7 inches) - Narrow, fuel-efficient size
65
65% aspect ratio = 127mm tall sidewall (comfortable ride)
R
Radial construction (standard modern tire)
15
15 inch wheel diameter - common compact car size
89H
89 = 1,279 lbs capacity • H = 130 mph max
Overall diameter: 25.0" • Great for fuel economy and city driving
EXAMPLE: FORD F-150 TRUCK
LT275/65R18 E
LT
LT = Light Truck tire (reinforced for heavy loads)
275
275mm width (10.8 inches) - Wide for stability and towing
65
65% aspect ratio = 179mm tall sidewall (towing stability)
R
Radial construction
18
18 inch wheel diameter - modern truck standard
E
Load Range E (10-ply rating) = 3,415 lbs capacity at 80 PSI
Overall diameter: 32.1" • Built for hauling and towing heavy loads
EXAMPLE: BMW M3 SPORTS CAR
245/35ZR19 93Y
—
No prefix = Euro-metric (slightly different load capacity vs P-metric)
245
245mm width (9.6 inches) - Wide for maximum grip
35
35% aspect ratio = 86mm sidewall (ULTRA low profile - sporty!)
ZR
Z = Speed capability above 149 mph • R = Radial
19
19 inch wheel - performance/luxury standard
93Y
93 = 1,433 lbs • Y = 186 mph max speed rating
Overall diameter: 26.0" • Performance-oriented, expensive, harsh ride
EXAMPLE: JEEP WRANGLER OFF-ROAD
33x12.50R15LT
33
33 inches = Overall diameter (flotation sizing format)
12.5
12.50 inches = Section width (very wide for off-road traction)
R
Radial construction
15
15 inch wheel diameter
LT
Light Truck rated for heavy loads and rough terrain
Metric equivalent: ~LT285/75R16 • Popular for 2.5-3" lifted Jeeps

Other Important Tire Markings to Know

📅

DOT Date Code

Look for "DOT XXXX XXXX 2319" on sidewall.

Last 4 digits = week and year manufactured
2319 = 23rd week of 2019
Replace tires over 6 years old regardless of tread depth!

❄️

Winter Tire Symbols

M+S = Mud + Snow (minimal winter capability)
3PMSF = Three-Peak Mountain Snowflake

3PMSF symbol means tire passed severe snow traction test. Required in some regions for winter driving.

⚡

Run-Flat Indicators

Look for: RFT, ROF, ZP, SSR, EMT, DSST

Run-flat tires can drive 50 miles at 50 mph after puncture. Require TPMS system and compatible wheels. Cannot be repaired in most cases.

🔢

Treadwear Rating

UTQG: Treadwear 500, Traction A, Temperature A

Treadwear 500 = lasts 2x longer than 250 rating (in theory). Traction A/B/C grades braking. Temperature A/B/C grades heat resistance.

⚖️

Max Pressure & Load

"MAX LOAD 1874 LBS @ 51 PSI COLD"

Maximum inflate pressure, NOT recommended! Check door jamb sticker for correct pressure (usually 32-42 PSI for cars, 60-80 PSI for LT trucks).

🌿

XL / Reinforced

May see XL, EXTRA LOAD, or REINFORCED

Extra load tires can carry more weight than standard load index. Common on heavy EVs (Tesla) and loaded vehicles. Requires higher inflation pressure.

⚠️ 5 Most Common Tire Size Reading Mistakes

📐
❌ Mistake #1: Thinking aspect ratio is an angle or degree
✓ Reality: It's a PERCENTAGE of the width! 65 means sidewall height is 65% of tire width, not 65 degrees.
🔢
❌ Mistake #2: Assuming rim diameter is in millimeters
✓ Reality: Width is in mm, but rim diameter is in INCHES! Mix-up causes ordering wrong tire size entirely.
⚠️
❌ Mistake #3: Ignoring load index and speed rating
✓ Reality: Downgrading these can cause tire failure! Always match or exceed OEM specs for safety.
⚖️
❌ Mistake #4: Believing P-metric and Euro-metric are identical
✓ Reality: Same numbers but different load capacity! P235/65R17 ≠ 235/65R17 in weight rating.
🚫
❌ Mistake #5: Mixing tire sizes on the same axle
✓ Reality: NEVER mix different sizes on same axle! Causes dangerous handling issues, illegal in most regions.

Quick Reference Cheat Sheet

Service Type Prefixes

PPassenger car
LTLight truck
STSpecial trailer
TTemporary spare

Speed Rating Guide

S112 mph
T118 mph
H130 mph
V149 mph
W168 mph
Y186 mph

Aspect Ratio Meanings

25-35: Ultra low profile (sports cars)
40-50: Low profile (performance sedans)
55-65: Standard (family cars/SUVs)
70-85: High profile (trucks/off-road)

Load Range (LT Tires)

C6-ply rating
D8-ply rating
E10-ply rating
F12-ply rating
💡 Pro Tip: Take a photo of your door jamb tire sticker before shopping!

Ready to Compare Tire Sizes?

Use our advanced tire size calculator to see exact differences between sizes, including diameter, speedometer error, and clearance requirements.

Launch Tire Size Calculator →View Tire Size Chart

Frequently Asked Questions

Where do I find my tire size?
Your tire size is molded into the sidewall of your current tires. Look for a series of numbers and letters like "P215/65R16 95H". You can also find the recommended tire size on: 1) Driver's door jamb sticker, 2) Glove box door, 3) Fuel filler door, 4) Owner's manual. The door jamb sticker is the most reliable source as it shows factory recommendations.
Can I put a different size tire on my car?
Yes, but with limitations. Alternative sizes must be within ±3% of your original tire's overall diameter (±2 mph speedometer error at 60 mph). You must also ensure: 1) Proper clearance (no rubbing on fenders/suspension), 2) Same or higher load index, 3) Same or higher speed rating, 4) Compatible rim diameter (18" tire only fits 18" wheel). Use our tire size calculator to compare sizes safely.
What happens if I use the wrong tire size?
Using incorrect tire size can cause serious issues: 1) Speedometer/odometer inaccuracy (affects resale value), 2) ABS and traction control malfunctions, 3) Rubbing on fenders or suspension (especially when turning), 4) Poor handling and reduced safety, 5) Transmission/drivetrain damage (especially AWD vehicles), 6) Voided vehicle warranty, 7) Failed vehicle inspection in some states. Always consult a professional if unsure.
Do all four tires need to be the same size?
Generally YES, except: 1) Staggered fitment cars (different front/rear from factory) like BMW M3, Porsche 911, Corvette - follow OEM specs exactly, 2) Temporary spare tire (T-rated donut) - emergency use only, max 50 mph, 50 miles. For AWD/4WD vehicles, all four tires MUST be within 2/32" tread depth of each other to prevent differential damage. Never mix sizes on the same axle (both fronts or both rears must match).
How do I know if a tire will fit my wheel?
The rim diameter (5th number) must match exactly. A 17" tire ONLY fits a 17" wheel - no exceptions. Additionally, tire width affects rim width compatibility: Each tire size has an approved rim width range (usually 1-2 inches variation). Example: 235mm tire typically fits 7.5"-9.0" wide rims. Consult tire manufacturer specifications or use our calculator for rim compatibility information.
What's the difference between 235/65R17 and P235/65R17?
The "P" prefix indicates P-metric (Passenger) sizing standard with specific load capacity calculations. Without "P" means Euro-metric sizing. Key differences: 1) Euro-metric tires can carry more weight than P-metric at same load index number, 2) Inflation pressure recommendations differ slightly, 3) P-metric more common in USA, Euro-metric in Europe/Asia. For safety, match your OEM tire specification exactly (with or without P prefix).