305/55R20 measures 33.2″ overall diameter and 12.0″ section width. Sidewall height is 6.6″, circumference is 104.3″, and the tire turns 607 times per mile. Those figures are geometry off the stamped size, not a brand or load range.
305/55R20 dimensions
| Measurement | Inches | Metric |
|---|---|---|
| Overall diameter | 33.2″ | 844 mm |
| Section width | 12.0″ | 305 mm |
| Sidewall height | 6.6″ | 168 mm |
| Circumference | 104.3″ | 2650 mm |
| Revolutions per mile | 607 | 377 / km |
The stamped width is 305 mm (12.0″). The aspect ratio is 55, so the sidewall is 55% of that width, which is 6.6″. The rim is 20 inches. Overall diameter is the rim plus two sidewalls: 20 + 2 × 6.6″ = 33.2″.
Sizes near 305/55R20
A swap that stays inside about 3% diameter usually keeps the speedometer honest and clears the same well. 33x12.50R20 is a close neighbour at -0.6% diameter. The percent column opens both stamps in the comparison calculator.
| Size | Diameter | Width | vs 305/55R20 |
|---|---|---|---|
| 33x12.50R20 | 33.0″ | 12.5″ | -0.6% |
| 275/60R20 | 33.0″ | 10.8″ | -0.7% |
| 285/60R20 | 33.5″ | 11.2″ | +0.8% |
| 295/55R20 | 32.8″ | 11.6″ | -1.3% |
| 275/75R17 | 33.2″ | 10.8″ | +0.1% |
| 295/70R17 | 33.3″ | 11.6″ | +0.2% |
How to read the stamped size
305/55R20 is a metric size: width in millimetres, aspect ratio as a percent of that width, then the rim diameter in inches after the R. The how to read tire size page walks the whole sidewall, including load index and speed rating, which this page does not use.
Diameter and width tell you whether a size is in the same family as what you have. They do not tell you whether 305/55R20 clears your fenders, matches your wheel width, or is load-rated for the vehicle. Check the door-jamb sticker and the wheel well before you buy.
Speedometer and revs
At 607 revolutions per mile, a trip computer calibrated for 305/55R20 is reading true only while this diameter is on the truck. A taller replacement under-reads speed and distance by the same percent the diameter grew. A shorter one over-reads. The comparison calculator puts a second size next to 305/55R20 and shows the error at 60 mph.