How to Read Tire Size
Every tire tells you its size on the sidewall. Here is the translation.
Tire size markings read as width in millimeters, aspect ratio, construction type, and rim diameter: 265/70R17 means 265 mm wide, a sidewall 70 percent of the width, radial construction, for a 17-inch rim. After the size come the load index and speed rating, such as 115T, which state how much weight the tire can carry and its maximum sustained speed.
The three sizing numbers
Width comes first in millimeters: 265 means the tread is 265 mm across. Aspect ratio is next: 70 means the sidewall height is 70 percent of the width, about 7.3 inches here. Rim diameter is last in inches: 17 means it fits a 17-inch wheel.
The letter between aspect and rim is construction: R for radial, which is nearly every modern tire. You may also see the size on the driver-door jamb placard, which lists what the vehicle shipped with.
Load index and speed rating
After the size, a code like 115T gives the load index (115 = 2,679 lb per tire) and speed rating (T = 118 mph sustained). These are minimums, not targets: never fit a lower load index than stock.
Higher speed ratings usually mean grippier, shorter-lived rubber. Match or exceed stock ratings unless you know exactly why you are deviating.
Where to find your size
Check the sidewall first, then the door jamb placard, then the owner's manual. All three should agree; if the tires on the car differ from the placard, a previous owner changed sizes.
When buying, the placard size is the safe default. Deviations are fine when you understand the tradeoffs, which is what the comparison calculator is for.
Skip the arithmetic
Decode your size and compare it on the calculator.
Reading tire size questions
What does 265/70R17 mean?
265 is the tread width in millimeters, 70 is the aspect ratio (sidewall is 70% of width), R means radial construction, and 17 is the rim diameter in inches.
What do the numbers after the size mean?
The code after the size, like 115T, gives the load index (how much weight each tire can carry, 115 = 2,679 lb) and the speed rating (T = 118 mph maximum sustained speed).
Where is my stock tire size listed?
All three list it. The door jamb placard shows the factory size and recommended pressures, which is the reference for any comparison.