Tyre size decoder
What the numbers mean, and what going bigger does to your speedo.
How it's worked out: overall diameter = rim (inches × 25.4) + two sidewalls (width × aspect %). Worn tyres read a few mm smaller. Bigger tyres beyond about 15 mm over the largest size your manufacturer lists usually need a modification check under the national guide most states and territories follow (VSB 14). Check with NT Motor Vehicle Registry and your insurer before you fit them. About vehicle standards.