How to Read Tire Size Numbers: Every Sidewall Code Explained
Every tire has its size molded into the rubber, but the string of letters and numbers can look more like a part number than a description. Once you know the order, a code like P225/65R17 95H tells you the tire's type, width, sidewall proportion, construction, wheel size, load capacity and speed category. Here's how to read each piece with confidence.
What a Tire Size Is and Where the Code Sits on the Sidewall
A tire size isn't one measurement. It's a standardized code that packs several dimensions and fitment details into a single line, so any tire maker or shop describes the same tire the same way. You'll find it in raised characters on the sidewall, usually near the brand and model name. It's typically molded on both sides, so it stays readable whichever way the tire is mounted. The same size appears on the tire placard inside the driver's door jamb and in the owner's manual as the factory recommendation.
Several other markings sit nearby, and people often mix them up. The DOT code is an identification number that ends with the week and year of manufacture. The UTQG ratings grade treadwear, traction and temperature performance. The maximum-pressure text gives a limit for the tire, not the pressure you should run day to day. None of these describe size. Throughout this guide, the example P225/65R17 95H is decoded one element at a time, so you can apply the same logic to the tires on your own vehicle.
Reading the Code Left to Right: Service Type, Width and Aspect Ratio
Start at the left. An optional leading letter gives the service type. P marks a passenger-car metric tire, LT a light-truck tire built for heavier duty, T a temporary spare and ST a trailer tire. When there's no letter, the size is usually Euro-metric, which follows the same dimensional logic but can be load-rated differently. The first number, 225 in our example, is the section width in millimeters. It's measured from sidewall to sidewall at the widest point, with the tire mounted and inflated on its specified rim width.
The number after the slash, 65, is the aspect ratio. It's the sidewall height as a percentage of section width, so it's a proportion rather than a direct measurement. A lower number means a shorter sidewall. Standing at the tire, you'd read the code aloud in order: P, 225, 65, R, 17, then 95H. Some tires also carry a second size marking in a different sizing system that looks nothing like the first. Use the marking whose format matches the one printed on your vehicle's placard.
Construction Letter and Rim Diameter: The Dimensions That Govern Fit
The letter after the aspect ratio describes the tire's internal construction. R stands for radial: the body plies run straight across the tire from bead to bead, roughly at a right angle to the direction of travel, with stabilizing belts under the tread. D means diagonal or bias-ply construction, where the plies cross at an angle, and B means belted bias. Radial construction is the norm on modern passenger vehicles. The next number, 17, is the rim diameter in inches. The tire's bead must match the wheel's diameter exactly to seat and seal.
Width, aspect ratio and rim diameter work together. Width and aspect ratio set the sidewall height, and two sidewalls plus the rim diameter give the overall tire diameter. That overall figure affects fender and suspension clearance. Because the speedometer effectively counts wheel rotations, a noticeably larger or smaller diameter can make the displayed speed differ from your actual speed. The rim diameter also says nothing about rim width. Each size is designed for a range of acceptable wheel widths, which you confirm from the tire maker's data, not from the sidewall code.
Load Index and Speed Rating: What the Service Description Means
The two characters after the size, 95H, form the service description. Unlike the numbers before them, they describe capability rather than dimensions. The load index, 95, points to a maximum load per tire on a standardized table, and a higher index means a higher rated load. The index isn't a weight itself, so 95 doesn't mean 95 pounds or kilograms. It also assumes correct inflation. An underinflated tire can't safely carry its rated load, and the rating never allows exceeding the vehicle's own payload or towing limits.
The speed rating letter, H here, puts the tire in a maximum sustained speed category on a separate standardized table. The rating comes from testing under controlled conditions. It isn't a recommendation to drive that fast, and damage, underinflation or overloading all reduce what the tire can handle in real use. Replacement tires should match or exceed both the load index and the speed rating on the vehicle placard. If a tire you're considering, or one already mounted, shows lower figures than the placard, have a tire professional check it before you rely on it.
Worked Example: What Each Number in P225/65R17 95H Means
Picture the code with a label under each piece. P marks a passenger tire, 225 is the width in millimeters, 65 is the aspect ratio, R means radial construction, 17 is the rim diameter in inches, 95 is the load index and H is the speed rating. In one plain sentence, this is a passenger radial tire 225 millimeters wide, with a sidewall 65 percent as tall as that width. It's built for a 17-inch wheel and carries load index 95 and speed rating H.
To get sidewall height, multiply the width by the aspect ratio: 225 times 0.65 is about 146 millimeters. The tire has a sidewall above and below the wheel, which adds roughly 293 millimeters. Add the 17-inch rim diameter, converted to the same units, and you have the overall diameter. Change only the aspect ratio, to P225/55R17, and the sidewall drops to about 124 millimeters, so the whole tire is roughly 45 millimeters shorter. Common mistakes include treating 65 as millimeters, reading 17 as a width and assuming 95 is a weight.
Using Tire Size Charts and Translators Without Getting Misled
A tire size chart or translator turns the sidewall code into more usable figures. It typically shows section width, calculated sidewall height and overall diameter, and it looks up the load index and speed rating in their standard tables. When you read a row, separate the direct decodes from the calculated numbers. Width, aspect ratio and rim diameter come straight off the sidewall. Sidewall height, overall diameter and sometimes circumference or revolutions per mile are calculations, and these nominal figures can differ slightly from a real tire's measured dimensions.
For the service description, look up the load index in its table to get the rated load per tire, then look up the speed letter in its own table. Don't expect the letters to run in alphabetical order: H, for example, falls between U and V, not right after G. The big limitation is that a translator tells you what a code means, not whether that size fits your vehicle. Fit depends on the placard, the wheel width and clearance. Before acting on any size that differs from the factory size, check the placard and ask a tire professional.