Complete Guide to Tire Sizes
Understanding tire sizes is essential for vehicle maintenance, safety, and performance. The alphanumeric codes on your tire sidewall contain crucial information about dimensions, load capacity, and speed ratings. This guide explains how to decode these numbers and choose the right tires for your vehicle.
Where to Find Your Tire Size
Your tire size is printed on the sidewall of your current tires. Look for a series of numbers and letters like "P225/65R17 102H". You can also find the recommended tire size in your vehicle's owner's manual, on a sticker inside the driver's door jamb, or inside the fuel door. Always use the manufacturer-recommended size unless you're specifically upgrading with professional guidance.
Decoding the Tire Size Code
Let's break down a common tire size: P225/65R17 102H. The "P" indicates a passenger car tire (LT means light truck). The "225" is the tire width in millimeters from sidewall to sidewall. The "65" is the aspect ratio—the sidewall height as a percentage of the width. In this case, the sidewall is 146mm tall (225 × 0.65 = 146.25mm).
The "R" stands for radial construction, which is standard for virtually all modern tires. Radial tires have ply cords that run perpendicular to the direction of travel, providing better traction, fuel economy, and ride comfort than older bias-ply tires. The "17" indicates the wheel diameter in inches—this tire fits a 17-inch rim.
Load Index and Speed Rating
The load index (102 in our example) indicates the maximum weight each tire can support when properly inflated. A load index of 102 means 1,874 pounds per tire. Four tires with this rating can support 7,496 pounds total. Never exceed your tire's load rating as this compromises safety and can cause tire failure.
The speed rating (H) indicates the maximum safe sustained speed. An H-rated tire is safe up to 130 mph. While you may never drive that fast, speed ratings also indicate a tire's ability to handle heat and maintain integrity at highway speeds. Higher-rated tires generally offer better handling and performance but may have firmer rides and shorter tread life.
Tire Width and Its Impact
Wider tires (higher first number) provide better traction and handling, especially in dry conditions. They have more contact with the road, improving cornering grip and braking performance. However, wider tires also cost more, reduce fuel economy slightly, and can be more prone to hydroplaning in wet conditions if tread design isn't optimized.
Narrower tires perform better in snow and ice because they apply more pressure per square inch of contact, helping cut through to the pavement. They're also lighter and create less rolling resistance, slightly improving fuel economy. For winter driving, many people switch to narrower tire sizes within manufacturer-approved ranges.
Aspect Ratio and Ride Quality
The aspect ratio dramatically affects ride quality and handling. Lower aspect ratios (like 45 or 50) mean shorter sidewalls, providing sportier handling with less flex during cornering. However, short sidewalls also transmit more road imperfections to the cabin, creating a firmer, sometimes harsher ride.
Higher aspect ratios (65, 70, or 75) indicate taller sidewalls that absorb bumps better, providing more comfortable rides. These tires are common on SUVs, trucks, and family sedans prioritizing comfort over sport handling. The taller sidewalls also provide more protection against wheel damage from potholes and curbs.
Plus-Sizing and Minus-Sizing
Plus-sizing means increasing wheel diameter while maintaining overall tire diameter by using lower-aspect-ratio tires. For example, changing from 205/60R16 to 225/45R17 keeps roughly the same overall diameter but provides sportier handling. This is popular for aesthetic and performance reasons but can compromise ride comfort.
Minus-sizing is the opposite—using smaller wheels with higher-aspect-ratio tires. This is common when installing winter tires, as it provides more cushion against winter potholes, reduces cost (smaller wheels and tires are cheaper), and can improve snow traction with narrower tires. Always stay within manufacturer-approved size ranges.
Load Range for Trucks
Light truck tires (starting with LT) include load range designations like C, D, or E. These indicate ply rating and maximum inflation pressure. Load Range C tires can inflate to 50 psi, D to 65 psi, and E to 80 psi. Higher load ranges support more weight but provide firmer rides. Choose load range based on your actual hauling and towing needs rather than maximum ratings.
Metric vs. Flotation Sizes
While most tires use metric sizing (like 225/65R17), some off-road and truck tires use flotation sizing (like 33×12.50R17). In this system, 33 is the overall tire diameter in inches, 12.50 is the width in inches, and 17 is the wheel diameter. Flotation sizing is more intuitive for enthusiasts who want specific tire heights for off-road capability or appearance.
Tire Replacement Guidelines
When replacing tires, all four should ideally be the same size, brand, and model for optimal handling and safety. If replacing only two tires, put the new ones on the rear axle regardless of drive type—this prevents oversteer and improves stability. Mixing tire sizes between front and rear is acceptable only if your vehicle's manufacturer specifies staggered sizing (common on sports cars).
Never install tires with different speed ratings or load indices than recommended by your vehicle manufacturer unless you've consulted with a tire professional. Lower ratings compromise safety, while unnecessary higher ratings waste money without meaningful benefits for normal driving. Stick to manufacturer specifications for optimal safety and performance.