The string of numbers and letters on your tire sidewall — something like 235/55R18 100H — looks like a code, but it's actually a straightforward description of the tire's size, construction, and ratings. How to read tire size is one of those basic things that every driver should know, especially if you ever need to call for a flat tire repair or replacement and someone asks "what size are your tires?" This guide breaks it down simply.
The Tire Size Code — Broken Down
235 — Tire Width in Millimeters
The first number is the tire's width in millimeters, measured from sidewall to sidewall when mounted on the proper rim. In this example, the tire is 235mm wide — about 9.25 inches. A wider tire provides a larger contact patch with the road, which can improve grip. A narrower tire cuts through snow and slush better.
55 — Aspect Ratio (Sidewall Height)
The aspect ratio is the sidewall height expressed as a percentage of the tire's width. In a 235/55 tire, the sidewall height is 55% of 235mm = approximately 129mm (about 5 inches). A lower number (like 40 or 45) means a shorter sidewall — the 'low-profile' look. A higher number (like 60 or 65) means taller sidewalls and more cushioning.
R — Radial Construction
The 'R' stands for Radial — the construction type of the tire. Almost every passenger and commercial tire made today is radial construction. You may occasionally see 'D' (diagonal/bias ply) on older vehicles, trailers, or off-road equipment, but R is the standard for all modern passenger vehicles.
18 — Rim Diameter in Inches
This is the diameter of the wheel rim the tire is designed to fit, measured in inches. An 18-inch tire goes on an 18-inch wheel — this must match exactly. Trying to mount an 18-inch tire on a 17-inch rim won't work. The rim size is what you need to give a tire shop or mobile service.
100 — Load Index
The load index is a numerical code that indicates the maximum weight a single tire can support when properly inflated. 100 = 1,764 lbs per tire. You must never install a tire with a lower load index than what your vehicle requires — it's a safety issue. The minimum load index for your vehicle is listed on the door jamb sticker inside the driver's door.
H — Speed Rating
The speed rating indicates the maximum sustained speed the tire is designed to handle. H = 130 mph. Most passenger car tires are rated H (130 mph) or V (149 mph). Performance tires may be W (168 mph) or Y (186 mph). You should match or exceed the original equipment speed rating — never install a lower-rated tire.
Where to Find Your Tire Size
There are three places to find your correct tire size:
Commercial Truck Tire Sizes — How They're Different
Commercial truck tires use a different but related sizing system. Common formats you'll see:
| Format | Example | What It Means |
|---|---|---|
| Metric commercial | 295/75R22.5 | Width 295mm / 75% aspect / 22.5" rim — common steer and drive tire for semis |
| Numeric/bias-style | 11R22.5 | Width ~11 inches / Radial / 22.5" rim — older format still common on drive/trailer tires |
| Low-profile commercial | 11R24.5 | 11-inch section width / 24.5" rim — highway long-haul application |
| Medium duty | 225/70R19.5 | Box trucks, delivery vans — smaller than semi tires, different rim diameter |
Tell Us Your Size — We'll Do the Rest
When you call Tire Dose for a flat repair or tire replacement, just read us the size from your sidewall — the full code like "235/55R18" or "295/75R22.5 for a commercial truck." We'll confirm we have the right tire in stock, quote you a price, and dispatch to your location in NJ, NYC, or Philadelphia PA.
Not sure? That's fine too — tell us your vehicle year, make, and model and we can look up the correct size for you.
Know Your Size? We'll Bring the Tire to You.
Mobile tire replacement for cars, SUVs & commercial trucks in NJ, NYC & Philadelphia PA. Quote in under a minute.
Call (862) 406-6404