Rim width and tyre fitment

The last number of a tyre size gives the rim diameter, but diameter alone is not enough. The rim width must also suit the tyre. Even a premium tyre fails early on the wrong width.

Reading a rim size

Tractor rims are usually marked like W15Lx38 or DW16x38. The leading letters give the rim profile (W, DW…), the middle number the width in inches, and the last number the diameter. W15Lx38 means a 15-inch-wide, 38-inch rim.

Recommended and permitted rims

Tyre makers publish a recommended rim and often several permitted rims for each size. The recommended rim gives the ideal profile; permitted rims are safe but change the shape slightly. Never fit a tyre outside these lists.

Rim too narrow

• Sidewalls are pinched inward and the tread becomes too rounded.
• The tread centre wears faster and traction drops.
• Extra strain at the bead area raises the risk of cracking.

Rim too wide

• Sidewalls are stretched and lose impact protection.
• The rim flange can cut the tyre on stones and stumps.
• Tubeless beads may not seat properly, causing air loss.

Going wider

Before switching to a wider tyre (for example a lower-series radial), check that the existing rim width suits it; often the new size needs a wider rim. Track width, fender clearance and front–rear ratio must also be rechecked. See how to read a tyre size.

Checklist

• Does the rim diameter match the last number of the tyre size?
• Is the rim width on the maker’s recommended or permitted list?
• Any cracks, dents, rust or weld marks on the rim?
• If fitting a tubeless tyre, is the rim tubeless-rated?