How to read tire size is a left-to-right walk of the sidewall stamp. On a 275/65R20 116T, the first four tokens are geometry and the last two are load and speed. The calculator on the homepage uses only the geometry. This page names every token so the door-jamb sticker and the tire agree.
The metric stamp, token by token
- 275. Section width in millimetres. Divide by 25.4 for inches: 10.8 in on this size.
- 65. Aspect ratio. The sidewall is 65% of the section width, so 178.75 mm or 7.0 in.
- R. Radial construction. Almost every passenger and light-truck tire sold today is an R. You will still see D (diagonal / bias) on some trailers and farm tires.
- 20. Rim diameter in inches. The wheel the tire is built for, not the overall diameter of the tire.
- 116. Load index. 116 is 2,756 lb per tire. The load index page has the table.
- T. Speed rating. T is 118 mph. That is a service limit, not a suggestion.
A prefix sometimes sits in front. P is P-metric (passenger). LT is light truck, usually a higher load range and a different inflation table. If there is no prefix, treat it as metric. An LT275/65R20 is not interchangeable with a P275/65R20 just because the geometry matches.
Flotation sizes read differently
Off-road and some truck tires use a flotation stamp: 33x12.50R15. The first number is already the overall diameter in inches. The second is section width in inches. The third is the rim. There is no aspect-ratio step. The converter will take either stamp.
| What you want | Metric example | Flotation example |
|---|---|---|
| Overall diameter | 34.1 in (calculated) | 33 in (stamped) |
| Section width | 275 mm / 10.8 in | 12.50 in |
| Rim | 20 in | 15 in |
Where to find the size on the vehicle
The tire has the size it was built to. The vehicle has the size it was designed for. Those can disagree after a previous owner "plus sized" the truck. Read both.
- Sidewall. Outboard, near the rim. Stamped, not painted.
- Driver door jamb. The placard lists the OEM size and the cold inflation pressure.
- Owner's manual. Same placard data, plus optional sizes the manufacturer approved.
If the tires on the truck and the placard disagree, the placard is what the speedometer was calibrated for. Put that size in the calculator as the current size, and the size you are considering as the new one.
What the calculator ignores, on purpose
Tread depth, brand, load range and whether the tire is new or worn all change the real-world diameter by a few tenths of an inch. A "33" that has been worn to 5/32 is no longer 33. This site reports the geometry of the stamp. Measure a standing tire if you need the height that is actually on the truck today.
The tire size meaning page is a shorter version of this one, aimed at the search that stops at "what does 275/65R20 mean." Come back here when you want the load index too.