How Tire Sizes Work:
Complete Guide to Every Number
Confused by tire markings like 225/45R18? This complete guide explains what every number means, how to choose the right size, and why it matters for your safety and vehicle performance.
Tire sizes seem complicated at first glance. But once you understand the system, it's actually quite logical. Every number and letter on your tire sidewall tells you something important about that tire's specifications, capabilities, and how it will perform on your vehicle.
Whether you're replacing tires, considering an upgrade, or just want to understand what your mechanic is talking about, this guide covers everything you need to know.
Understanding Tire Size Numbers
225 / 45 R 18 94H
This is a standard tire size marking. Each component tells you something specific about the tire. Let's break it down:
225 = Tire Width
The first number (225) is the tire width in millimeters. This is the measurement across the tire from sidewall to sidewall when mounted on a standard wheel.
Common widths: 175, 185, 195, 205, 215, 225, 235, 245, 255, 265, 275, 285, 305
45 = Aspect Ratio
The second number (45) is the aspect ratio, expressed as a percentage. It's the ratio of the tire's height to its width.
For 225/45: Sidewall height = 225mm × 0.45 = 101.25mm
Lower ratio = lower profile: 35 aspect ratio = sportier, stiffer ride
Higher ratio = taller sidewall: 70 aspect ratio = more comfortable, better off-road
R = Radial Construction
The letter R indicates the tire is a radial tire. This is the standard construction for all modern passenger car tires.
You'll rarely see anything else: Unless you own a classic car, your tires will be radial (R).
18 = Wheel Diameter
The number 18 is the wheel diameter in inches. This is the size of the rim that the tire fits on.
Common sizes: 13", 14", 15", 16", 17", 18", 19", 20", 21", 22"
Note: Larger wheels typically come with lower aspect ratios for sportier appearance.
94H = Load Index & Speed Rating
The number (94) and letter (H) at the end tell you the tire's load capacity and maximum speed rating.
H = Speed Rating (maximum safe speed of 210 km/h or 130 mph)
Load Index vs Load Range (Critical Difference!)
This is one of the most important distinctions for tire selection. Load Index is used for passenger cars, while Load Range is used for trucks. They measure the same thing (load capacity) but use different systems.
📊 Load Index (Cars)
A two-digit number (88, 91, 94, 97, etc.) that indicates the maximum weight the tire can safely carry.
| Index | Capacity |
|---|---|
| 88 | 560 kg (1,235 lbs) |
| 91 | 615 kg (1,356 lbs) |
| 94 | 670 kg (1,477 lbs) |
| 97 | 730 kg (1,609 lbs) |
Multiply the load per tire × 4 to get total vehicle capacity
🏋️ Load Range (Trucks)
A single letter (C, D, E, F) that indicates ply rating and maximum inflation pressure.
| Range | Ply | Max PSI |
|---|---|---|
| C | 6-ply | 50 PSI |
| D | 8-ply | 65 PSI |
| E | 10-ply | 80 PSI |
| F | 12-ply | 95 PSI |
💡 Key Difference
Never mix these systems. If your vehicle requires Load Range E, using a tire with only Load Index 94 would be unsafe—it doesn't have enough capacity. Conversely, using Load Range E tires on a car designed for Load Index 91 would feel rough and harsher to ride.
Speed Ratings Explained
The final letter on your tire (like the H in 225/45R18 94H) is the speed rating. It indicates the maximum speed the tire is designed to safely sustain.
| Rating | Max Speed (km/h) | Max Speed (mph) | Best For |
|---|---|---|---|
| T | 190 | 118 | Economy cars |
| H | 210 | 130 | Most sedans & SUVs |
| V | 240 | 149 | Performance cars |
| W | 270 | 168 | High-performance |
| Y | 300+ | 186+ | Track/exotic cars |
⚠️ Important Note
The speed rating doesn't mean your tire is only good up to that speed—it means it's designed and tested to be safe AT that speed. However, always follow local speed laws and drive safely regardless of tire speed rating.
Rolling Resistance & Fuel Efficiency
Rolling resistance is the friction between tire and road as the tire rolls. It directly affects fuel economy and emissions. This is especially important for electric vehicles and hybrids.
Understanding Rolling Resistance
🔢 Rolling Resistance Coefficient (RRC)
RRC values range from 6.0 (very low, maximum efficiency) to 11.0+ (high resistance, poor efficiency). Every 0.1 point increase in RRC equals approximately 0.5% loss in fuel economy.
⚡ Impact on Fuel Economy
A car getting 30 MPG with low-RRC tires (6.5) could drop to 25 MPG with high-RRC tires (9.5)—a loss of 5 MPG. Over a year, that could cost $300-600 extra in fuel.
🔋 EV Impact (Critical!)
For electric vehicles, rolling resistance is even more critical. Every 1.0 point difference in RRC can equal 3-4% difference in electric range. On a 300-mile EV, that could mean 10-15 miles of range difference.
✅ Low RRC (6.5-7.5)
Maximum fuel economy, great for hybrids and EVs. Brands: Michelin, Continental, Bridgestone (special lines).
⚠️ Medium RRC (7.5-8.5)
Balanced performance and efficiency. Most standard all-season tires fall here.
❌ High RRC (8.5-11.0+)
Poor efficiency, avoid if economy matters. Includes all-terrain and mud-terrain tires.
UTQG Rating: Wear, Traction & Temperature
You might also see UTQG ratings on tires. UTQG stands for Uniform Tire Quality Grading and appears as numbers like "A A 500" on the tire sidewall.
500 = Treadwear Rating
This number indicates the tire's longevity. A rating of 500 means the tire should last roughly 50,000 miles under normal conditions (though actual life depends on driving habits, maintenance, and road conditions).
Traction Rating (A, B, or C)
Measures the tire's ability to stop on wet pavement. A is best, C is minimum legal requirement.
Most modern all-seasons are A-rated.
Temperature Rating (A, B, or C)
Measures the tire's heat resistance. A-rated tires handle heat better and are safer at sustained high speeds.
Important for high-performance vehicles and extended highway driving.
Frequently Asked Questions
Can I use a different tire size than my OEM spec?
What's the difference between all-season and summer tires?
How often should I rotate my tires?
What PSI should I use?
Do I need to match tire brands?
Ready to Choose Your Tires?
Now that you understand tire sizes and specifications, use our free tire size calculator to find the perfect tires for your vehicle. Enter your vehicle details and compare options side-by-side.