Tractor tire size conversion
Type your tractor tire size — old inch size or metric. See direct equivalents and same-height tires in other widths.
How the conversion works
- The tire and tractor industries group farm drive tires by Rolling Circumference Index (RCI): tires in the same group have about the same diameter and rolling circumference.
- Direct equivalent = same RCI group, same width class and same rim diameter, e.g. 18.4R38 ↔ 480/80R38.
- Same height, other width = same RCI group, narrower or wider — used to change row width or flotation without changing front axle lead.
- Bias (18.4-38) and radial (18.4R38) of the same code are the same nominal size; radials are usually a little taller under load.
- Before buying: check rim width, load index at your pressure, and clearance — and your tractor maker's approved list.
Source: Firestone Ag data book, Rolling Circumference Index chart (approximate per group). Diameters for metric sizes: Continental Agricultural Tires Technical Databook where available. Checked October 2026.