Tire size meaning is the three numbers on the stamp. Width, aspect ratio, rim. Everything else on the sidewall (load index, speed rating, M+S, date code) is a different question. If you want the long walk, use how to read tire size. This page is the geometry.
The three numbers
| Token | Means | On this size |
|---|---|---|
| 275 | Section width in mm | 10.8 in |
| 65 | Sidewall as % of width | 7.0 in sidewall |
| R | Radial | Radial |
| 20 | Rim diameter in inches | 20 in |
| (result) | Overall diameter | 34.1 in |
Overall diameter is not printed as a fourth number on a metric tire. You calculate it: rim + 2 × sidewall. That is why people search "275/65R20 in inches." The 275/65R20 page is that result. The tire size calculator does it for any stamp.
Why the same width is not the same tire
275/65R20 and 275/55R20 share a width and look related on a shelf. They are not the same height. Dropping the aspect ratio from 65 to 55 on a 20-inch rim cuts the sidewall and the diameter: 34.1 in down to 31.9 in. That is a speedometer error of about 6.5%, and a look that does not match the well the 65 was designed for.
The aspect ratio page is this idea on its own. Plus sizing is the deliberate version: you drop the aspect ratio because you raised the rim, and you are trying to land back near the original diameter.
Slang vs stamp
People say "33s" and "35s." Those are flotation diameters, or a loose height band. A 275/65R20 is 34.1 inches. Calling it a 33 is common and wrong. The 33 vs 35 page keeps the slang and the stamps on the same table.
Knowing what the stamp means does not tell you the tire fits the truck. Fitment is wheel width, offset, load rating and the well. Read the door-jamb placard before you treat a decoded size as a shopping list.