Tire Size Guide: Read Your Sidewall Right

A tire sidewall can look like a string of random letters and numbers when you need a replacement fast: 265/70R17 115S, LT285/75R16, 225/45ZR18. But that code tells you whether a tire will fit your wheel, carry the load, clear the suspension, and perform the way your vehicle needs it to. This tire size guide breaks it down so you can shop with confidence, whether you drive a commuter car, work truck, SUV, trailer, or weekend off-road rig.

Start With the Size on Your Current Tire

The fastest place to find your tire size is the sidewall of the tire currently on your vehicle. You can also check the driver-side door-jamb placard, owner’s manual, or the original equipment specifications for your vehicle. The door placard is especially useful because it typically lists the factory-recommended tire size, cold inflation pressure, and sometimes separate front and rear sizes.

For a standard passenger or SUV tire, you may see a marking such as 225/65R17 102H. Each section has a job. The first three numbers identify the tire’s width. The number after the slash tells you the sidewall height as a percentage of that width. The letter and next number identify the construction and wheel diameter. The final load and speed ratings tell you how much weight the tire can support and the speed capability it was designed to handle.

Do not order based on wheel diameter alone. A 17-inch wheel can use many different tire sizes, and the wrong width or overall diameter can affect clearance, speedometer accuracy, handling, and load capacity.

Tire Size Guide: What Every Number Means

Let’s use 225/65R17 102H as the example.

225: Section width in millimeters

The first number, 225, is the nominal width of the tire from sidewall to sidewall in millimeters. In this case, it is about 8.9 inches wide. A wider tire can provide a larger contact patch and a more aggressive look, but it can also be heavier, more expensive, and more likely to rub if the wheel offset or suspension clearance is wrong.

Width is not an exact promise of tread width. Different tire brands and models can measure slightly differently, especially when mounted on wheels of different widths. That matters when you are building a custom truck, lowering a car, or fitting larger off-road tires.

65: Aspect ratio, or sidewall height

The 65 means the sidewall height is 65% of the tire’s width. On a 225-mm-wide tire, that equals roughly 146 mm of sidewall height.

A higher aspect ratio, such as 70 or 75, generally means more sidewall. That can improve ride comfort and provide useful flex for off-road driving. A lower aspect ratio, such as 40 or 35, creates a shorter sidewall for sharper steering response and a performance-focused appearance, but ride quality can be firmer and wheels have less protection from potholes.

R: Radial construction

The R stands for radial construction. Nearly every modern passenger, SUV, light-truck, and performance tire uses radial construction, so this letter is common. You may occasionally see other construction markings on specialty, agricultural, trailer, or industrial tires, where matching the original specification is especially important.

17: Wheel diameter in inches

The 17 means the tire is made for a 17-inch wheel. This measurement must match your wheel exactly. A tire designed for an 18-inch wheel cannot be safely installed on a 17-inch wheel, even if the rest of the numbers appear close.

Wheel diameter changes are possible when you build a matched tire-and-wheel package. For example, moving from a 17-inch to an 18-inch wheel often requires a lower-profile tire to keep the overall diameter close to stock. That is called plus sizing. It can improve appearance and steering feel, but it also brings trade-offs in ride comfort, tire cost, and damage resistance.

102: Load index

The load index is a coded number representing how much weight each tire can carry at its rated pressure. A load index of 102 supports up to 1,874 pounds per tire. Four tires do not always mean four times that number is available payload, since vehicle axle ratings, inflation pressure, and weight distribution still control what your vehicle can safely carry.

For replacement tires, meet or exceed the original equipment load index. This is critical for loaded SUVs, towing vehicles, commercial vans, trailers, and trucks that see jobsite duty. Choosing a lower load rating to save money is not a shortcut worth taking.

H: Speed rating

The H is the speed rating. It indicates the tire’s tested capability under controlled conditions, not a suggested driving speed. H-rated tires are rated up to 130 mph. Other common ratings include S, T, V, W, and Y.

Speed ratings also connect to handling, heat resistance, and the intended character of the tire. A performance car may require a V-, W-, or Y-rated tire, while many all-terrain or highway truck tires use S or T ratings. Stay at or above your vehicle manufacturer’s required rating unless a qualified fitment expert recommends a specific exception for the vehicle’s intended use.

Understanding LT, P-Metric, Flotation, and Metric Sizes

Not every tire follows the same format. The letters before the size tell you a lot about the tire’s intended job.

A P prefix, such as P235/75R15, means P-metric passenger tire. These are common on cars, crossovers, minivans, and many light-duty SUVs. An LT prefix, such as LT265/70R17, means Light Truck. LT tires are built for heavier loads, tougher service, and often higher inflation pressures. They are a strong option for trucks that tow, haul, run rough roads, or need extra durability, though they may ride firmer and weigh more than P-metric options.

Off-road and truck shoppers will also see flotation sizes such as 35x12.50R20LT. The first number is overall tire diameter in inches, the second is width in inches, and the final number is wheel diameter. A 35x12.50R20 is approximately 35 inches tall, 12.5 inches wide, and fits a 20-inch wheel.

Trailer tires often use ST before the size, meaning Special Trailer. Trailer tires are engineered for trailer service and should not be substituted with regular passenger tires just because the size appears similar. Match the correct ST size, load range, and inflation requirement for safe hauling.

Load Range Matters for Trucks and Trailers

Along with the load index, LT and trailer tires may carry a load range, such as C, D, E, F, or G. This designation reflects the tire’s load-carrying and inflation capability. Load Range E is common on heavy-duty pickups and trailers, but it is not automatically the right answer for every truck.

A higher load range can add confidence for towing and carrying equipment, yet it can also create a firmer unloaded ride. The right choice depends on how you use the vehicle. A daily-driven half-ton truck with occasional light hauling may not need the same tire construction as a work truck towing heavy equipment every week.

Can You Change Tire Sizes?

Yes, but changing tire size should be a fitment decision, not a guess. A slightly taller tire can increase ground clearance and fill out a truck or SUV wheel well. A wider tire can improve stance and traction in some conditions. On the other hand, even a modest size change may cause rubbing at full steering lock or suspension compression.

Before moving away from factory sizing, consider overall diameter, wheel width, wheel offset, brake clearance, suspension lift or leveling height, and the tire’s actual measured dimensions. Larger tires can also alter effective gearing, reduce fuel economy, affect acceleration, and cause the speedometer to read lower than your actual speed. On newer vehicles, major diameter changes can affect driver-assistance systems and may require recalibration.

For performance vehicles, keeping the rolling diameter close to factory specification is especially important. For lifted trucks, advertised tire size alone is not enough. One brand’s 35-inch mud-terrain may measure differently than another brand’s 35-inch all-terrain once mounted and inflated.

Match the Tire to the Road Ahead

Correct sizing gets the tire on the vehicle. Choosing the right tire type makes the purchase work for your life.

All-season tires are a practical choice for many daily drivers that see rain, dry pavement, and mild winter conditions. Touring tires focus on comfort, road manners, and tread life. Performance tires prioritize grip and response, often with shorter tread life and less capability in cold conditions. All-terrain tires balance highway use with dirt, gravel, and trail traction, while mud-terrain tires are built for aggressive off-road conditions and can be louder on pavement.

If you tow, carry tools, drive long highway miles, or work in demanding conditions, prioritize load capacity, durability, and the correct inflation range. If your goal is a wheel-and-tire upgrade, make sure the look you want still leaves room for safe turning, braking, and suspension travel.

When the sidewall code is clear, shopping gets easier. Start with the vehicle placard, match the required wheel diameter and load capacity, then choose the tread design that fits your road, your workload, and your next trip. If a size looks close but you are not fully sure, get fitment help before ordering - the right tire should feel ready for the road, not like a gamble.