How to Read Tire Sizes: What the Numbers on Your Sidewall Mean
To check your tire size, look at the sidewall for a code like 275/55R20 117T — that's width in mm, aspect ratio as a percentage, radial construction, wheel diameter in inches, load index, and speed rating in that order. Every letter and number on the sidewall tells you something specific about fit, load capacity, and performance.
Every tire sold in the US carries the same standardized sidewall code, but most drivers never learn to read it. That's fine until you need to replace a set, run a plus-size, or pick tires for a lifted truck. Once you can decode the sidewall, you can size, source, and swap tires with confidence — and avoid the surprisingly expensive mistake of ordering the wrong spec.
The Core Code: 275/55R20 117T
Take a common truck tire: 275/55R20 117T. Each segment answers a specific question about the tire.
| Segment | Meaning | Example |
|---|---|---|
| 275 | Section width in mm (tread + sidewall bulge) | 275 mm ≈ 10.8" |
| 55 | Aspect ratio: sidewall height as % of width | 0.55 × 275 = 151 mm sidewall |
| R | Construction: R = radial (nearly universal today) | R |
| 20 | Wheel diameter in inches | 20" wheel |
| 117 | Load index (max load per tire) | 117 = 2,833 lb |
| T | Speed rating (sustained top speed) | T = 118 mph |
Section Width and Recommended Rim Width
Section width is measured on a specific "measuring rim" width chosen by the tire maker. Mount that same tire on a wider or narrower rim and its actual mounted width shifts by roughly 5 mm per half-inch of rim width change. Every tire size has an approved wheel-width range printed on the spec sheet — a 275-width tire typically fits rims from about 8.5" to 10.5" wide.
Mount that same 275 on a 12" rim and you get a stretched tire — popular for stance builds, but hard on sidewalls and often illegal for track days. Mount it on a 7" rim and the sidewall balloons out, killing steering response.
Aspect Ratio and Overall Diameter
Aspect ratio is sidewall height as a percentage of section width. A 275/55 tire has a sidewall of 275 × 0.55 = 151 mm. Overall tire diameter is:
For that same 275/55R20: 20 + (2 × 275 × 0.55 ÷ 25.4) = 31.9". Change diameter by more than about 3% and your speedometer starts reading noticeably wrong, ABS timing shifts, and (on four-wheel-drive vehicles) transfer cases can chatter.
Load Index and Speed Rating
Load index is a lookup number for how much weight one tire can carry at max pressure. Common values:
| Load index | Max load per tire | Typical use |
|---|---|---|
| 91 | 1,356 lb | Compact sedan |
| 100 | 1,764 lb | Midsize sedan / SUV |
| 117 | 2,833 lb | 1/2-ton truck |
| 126 | 3,748 lb | 3/4-ton truck / heavy tow |
Speed ratings run S (112 mph), T (118), H (130), V (149), W (168), Y (186). Always match or exceed the rating on your door jamb — downgrading is a liability issue and can void insurance in some states.
P, LT, and XL — Which Tire Category Do You Need?
- P-metric (or no prefix): passenger. Lightest construction, best ride, lowest load rating.
- LT-metric: light truck. Stiffer sidewalls, higher load, better for towing and off-road.
- XL / Extra Load: heavier-duty passenger tire. Requires higher inflation.
- Euro-metric (no P or LT): uses SAE load tables. Read the spec sheet before crossing between systems.
DOT Code and Date of Manufacture
The DOT stamp ends with a four-digit date code: DOT ... 2325 means the 23rd week of 2025. Tires older than six years should be inspected regardless of tread depth; rubber oxidizes even if the tire hasn't rolled.
Plus-Sizing Safely
Plus-one and plus-two sizing (going up a wheel diameter while dropping aspect ratio to keep overall diameter close) is the standard way to fit larger wheels. Aim to stay within 3% of stock overall diameter. Anything larger and you should recalibrate the speedometer, re-check gearing, and confirm fender clearance at full lock and full compression.