TL;DR
- A tire size like 245/40R18 97Y breaks into five parts: section width (245 mm), aspect ratio (40%), construction type (R = radial), rim diameter (18 inches), and service description (97Y = load index + speed rating).
- Never buy a tire with a lower load index or speed rating than what your door jamb sticker specifies. Both are safety minimums, not preferences.
- The DOT date code tells you when the tire was made. The last four digits show week and year - avoid tires older than 4-6 years regardless of tread depth.
- UTQG gives you a treadwear estimate, traction grade, and temperature grade. Higher treadwear numbers mean longer-lasting rubber, but compare within the same tire category for accurate comparisons.
- TireOrbit shows section width, aspect ratio, rim diameter, load index, speed rating, UTQG, and DOT year on every listing. Search by vehicle, size, or browse all tires.
The full tire size code: what it looks like
Most passenger tires follow this format: P245/40R18 97Y. Some tires omit the P prefix. Some SUV and light-truck tires start with LT. Strip away the prefix and you are left with the same structure: width / aspect ratio + construction type + rim diameter, followed by a service description.
Walk up to your tire right now and you will find this code stamped in large characters on the sidewall. It is also on the door jamb sticker on the driver-side door frame. The door jamb sticker is your authoritative source - your current tires may not be the original size if a previous owner made changes.
According to NHTSA's tire safety guidelines, using a tire outside your vehicle's specified size range can affect braking distance, handling, speedometer accuracy, and load capacity. It is not a cosmetic decision.
Section width: the first number
The first number in the size code is the section width in millimeters. On a 245/40R18, the tire is 245 mm wide at its widest point - measured from the outer sidewall to the inner sidewall when mounted on the specified rim width and inflated to the correct pressure.
Common widths run from 155 mm on small economy cars up to 315 mm on high-performance vehicles. Wider tires give more contact patch with the road, which generally improves cornering grip but can hurt fuel economy and hydroplaning resistance in heavy rain.
One practical note: rim width affects how the tire actually sits. A wide tire on a narrow rim rounds out; a narrow tire on a wide rim stretches. Both conditions are outside the tire's design parameters and can affect handling and wear. Tire manufacturers publish a rim width range for each tire model - the middle of that range is usually ideal.
Aspect ratio: the second number
The aspect ratio is the number after the slash, expressed as a percentage. It describes the sidewall height as a proportion of the section width.
On a 245/40R18: the sidewall height is 40% of 245 mm = 98 mm. That is roughly 3.9 inches of sidewall on each side of the wheel.
Low aspect ratios (35, 40, 45) mean short stiff sidewalls. You get sharper steering response and a sportier look, but the ride is firmer and the tire is more vulnerable to pothole damage. High aspect ratios (65, 70, 75) mean taller softer sidewalls. Ride comfort improves, the tire absorbs more road shock, but steering response slows slightly.
Trucks and SUVs often run 65 or 70 series tires because they carry more weight and benefit from the additional sidewall cushion. Sports cars run 35 or 40 series for lateral stiffness in corners.
Construction type: the letter before the rim diameter
The letter between the aspect ratio and the rim diameter tells you how the tire was built. R means radial construction, which is what virtually every passenger car, SUV, and light-truck tire uses today. Radial tires have steel belt layers running perpendicular to the direction of travel, giving consistent contact with the road.
You may occasionally see D (diagonal/bias-ply) on vintage vehicles, trailers, or some off-road tires. Bias-ply construction layers the cords at an angle across the tire. They handle heavier loads and impacts well but run hotter and wear faster than radials at highway speeds.
The U.S. Tire Manufacturers Association has a useful primer on tire construction if you want to go deeper on this topic.
Rim diameter: the number after the R
This is the wheel diameter in inches that the tire fits. A tire marked R18 mounts only on an 18-inch wheel. This number must match exactly. There is no flexibility here.
If you want to change your rim size - say, going from 17-inch to 18-inch wheels - you need to recalculate the rest of the tire size to keep the overall diameter within roughly 3% of the factory size. Otherwise your speedometer will read incorrectly and your transmission shift points will be off. Many tire shops and fitment guides offer a "plus sizing" calculator for this.
Larger wheels generally look better and allow for larger brake calipers, but they also force lower aspect ratios, which makes the ride harsher and increases the cost of tire replacements.
Load index: how much weight the tire can carry
The service description after the rim diameter starts with the load index. On a 97Y rating, 97 is the load index. Each number maps to a maximum load capacity in pounds:
- 91 = 1,356 lbs
- 94 = 1,477 lbs
- 97 = 1,609 lbs
- 100 = 1,764 lbs
- 105 = 2,039 lbs
Your door jamb sticker shows the minimum load index for your vehicle. The total load capacity of all four tires must exceed the vehicle's gross vehicle weight rating (GVWR). Do not go below the specified load index. A tire that cannot handle the weight will overheat, deform, and eventually fail.
NHTSA reports that tire failure from overloading is a significant factor in tire-related crashes each year. Load index is not a number to treat casually.
Speed rating: the letter at the end
The letter following the load index is the speed rating. It indicates the maximum sustained speed the tire can handle under its rated load. Common ratings:
- S = 112 mph - family sedans, minivans
- T = 118 mph - standard passenger cars
- H = 130 mph - sport sedans, some SUVs
- V = 149 mph - performance cars
- W = 168 mph - high-performance vehicles
- Y = 186 mph - sports cars
Speed rating also reflects the tire's internal construction quality and heat resistance. A higher-rated tire handles heat buildup better, which matters even if you never drive at those speeds. You can always use a tire with a higher speed rating than required. Never use one with a lower rating - it compromises the tire's ability to shed heat at highway speeds.
DOT code: when was the tire actually made?
Every tire sold in the US carries a DOT code - a series of letters and numbers stamped on the sidewall starting with the letters "DOT." The last four digits of this string are the ones to pay attention to. They represent the week and year of manufacture.
A code ending in 2319 means the tire was made in the 23rd week of 2019. A code ending in 0424 means the 4th week of 2024.
Why does this matter? Rubber compounds degrade over time regardless of how much tread remains. The oils and chemicals that keep rubber pliable slowly evaporate. Older tires can look fine but have reduced grip and increased cracking risk under load. The U.S. Tire Manufacturers Association recommends that tires 10 or more years old - from manufacture date, not purchase date - should be removed from service. Many manufacturers recommend replacement at 6 years. If you are buying closeout tires or used tires, always check the DOT date first.
UTQG: treadwear, traction, and temperature
UTQG stands for Uniform Tire Quality Grading. The US government requires this rating system on most passenger tires. It gives you three pieces of information:
Treadwear number
A number like 400, 500, or 700. This is a relative rating - a tire rated 400 is expected to last twice as long as a reference tire rated 200 under controlled test conditions. Higher is better for longevity. Performance tires and winter tires often carry lower treadwear numbers because their compounds prioritize grip over longevity. Touring tires often hit 600-800 because they use harder, longer-lasting compounds.
Traction grade
Graded AA, A, B, or C. This measures how well the tire stops on wet asphalt and wet concrete in a controlled straight-line braking test. AA is the best. Most good all-season tires are rated A or AA. A traction grade of C means the tire stops noticeably worse on wet pavement. Given that wet-road stopping distance directly affects crash avoidance, this is not a grade to ignore.
Temperature grade
Graded A, B, or C. This measures the tire's ability to resist and dissipate heat at speed. An A-rated tire handles heat buildup better than a C-rated tire. All tires sold in the US must achieve at least a C temperature grade to meet federal minimum standards. Most performance and touring tires are rated A.
P-metric vs. LT vs. no prefix: what the prefix means
Passenger tires often begin with P (P-metric), following a US sizing standard. Some tires omit the P entirely and use European metric sizing - these are sometimes called "Euro-metric" tires. The sizes look the same (e.g., 245/40R18) but P-metric and Euro-metric tires have slightly different load capacity calculations. When mixing these, check the manufacturer's fitment guide.
LT (Light Truck) tires are built for heavier loads and often come with higher ply ratings. If your truck or large SUV specifies LT tires, do not substitute P-metric tires - the load capacity will be lower than the vehicle needs.
Some tires carry a C suffix (e.g., 215/65R16C) indicating commercial van fitment - a higher load range in a passenger tire form factor.
Reading the full code: a practical example
Take a tire stamped P225/65R17 102T:
- P - passenger vehicle tire
- 225 - 225 mm section width
- 65 - sidewall height is 65% of 225 mm = 146 mm
- R - radial construction
- 17 - fits a 17-inch rim
- 102 - load index 102 = 1,874 lbs per tire
- T - speed rating 118 mph
This is a typical size for a mid-size SUV or crossover. A tire in this size with a UTQG of 700 A A is a long-wearing touring tire with good wet traction and solid heat resistance - a strong choice for a daily-driver family vehicle.
Where to find your correct tire size
There are three reliable places to find your vehicle's required tire size:
- Driver-side door jamb sticker - this is the most reliable source. It lists the OEM tire size, the minimum load index, and the correct inflation pressure for front and rear.
- Owner's manual - the specifications section lists all approved tire sizes, including alternates the manufacturer may have tested.
- Current tires - readable from the sidewall, but only accurate if the tires have never been changed from stock.
On TireOrbit, search by your vehicle's year, make, and model and the site returns only sizes that fit your specific trim. Or if you already know your size, go directly to search by size.
Using this knowledge to shop smarter
Most people replace tires by telling a shop "I drive a 2021 Honda CR-V" and letting the counter staff look it up. That works, but it leaves you dependent on one person's knowledge and upsell choices. When you understand the numbers yourself, you can:
- Confirm the tire the shop is recommending actually matches your load index and speed rating requirements.
- Compare UTQG treadwear numbers across tire models to estimate long-term cost per mile.
- Check the DOT date code on any tire before it is mounted, especially on clearance or closeout inventory.
- Verify the load index supports your vehicle's weight before buying tires for a loaded truck or SUV.
Tire Rack's independent test database lets you filter by size and compare wet braking, dry handling, noise, and tread-life scores across dozens of models. It is one of the most useful free tools available for tire research.
Shopping on TireOrbit
Every TireOrbit listing shows the full size, load index, speed rating, UTQG grades, and DOT year - not buried in a spec sheet, but right on the product card. Prices include a $100 install credit redeemed at a local installer of your choice. No hidden mount, balance, or disposal fees.
Start by searching your vehicle for OEM-correct fitments, use the by-size search if you already know your size, or browse all tires and filter by category, brand, or UTQG rating.