Tire aspect ratio is the sidewall height as a percent of the section width. On 275/65R20 the sidewall is 65% of 275 mm. Change that 65 and you change overall diameter twice (once at the top of the tire, once at the bottom) even if the width and rim stay put.
The formula
Sidewall (mm) = width × (aspect / 100). Overall diameter (in) = rim + 2 × sidewall in inches. A 10-point drop in aspect ratio on a 275 mm tire is 27.5 mm less sidewall, 2.2 inches less diameter. That is not a rounding error. That is a different tire.
| Size | Sidewall | Diameter |
|---|---|---|
| 275/65R20 | 7.0 in | 34.1 in |
| 275/60R20 | 6.5 in | 33.0 in |
| 275/55R20 | 5.9 in | 31.9 in |
| 275/50R20 | 5.4 in | 30.8 in |
Why plus sizing exists
If you raise the rim, you have to drop the aspect ratio to keep diameter close to stock. That is plus sizing: more wheel, less sidewall, similar overall height, a sharper turn-in and a harsher ride. The plus sizing page works a full example. The calculator is the check: put stock in A and the plus-size in B and read the diameter percent.
A low aspect ratio is not "more tire." It is less sidewall. Off-road, that is usually the wrong direction. On a highway sedan, it is how 18 and 19 inch wheels still fit in a well designed around a 16.
What a "65-series" actually feels like
The series number is the aspect ratio. A 65-series tire has more air and more flex than a 45-series of the same width. That flex is comfort on broken pavement and, past a point, vague steering. There is no universal "best" series. There is the series the vehicle was designed around, and the series you are considering, and the diameter difference between them.
A 70-series all-terrain is taller than a 55-series highway tire of the same width. It is not automatically a mud tire. Tread, load range and construction are separate stamps.