Tread depth: when to replace agricultural tyres

A car tyre reaches a legal minimum and gets replaced. An agricultural tyre fades. As the lugs wear down, the tractor slips a little more, the driver opens the throttle a little further, and the extra diesel and the lost hours never appear on any invoice. By the time the tyre looks worn, it has usually been costing money for a season or two.

How to measure lug height

Use a tread depth gauge or a steel rule, and measure from the base of the tread to the top of the lug. Take the reading at the centre of the lug, not at the shoulder, and take it on at least four lugs spaced around the tyre — uneven wear is common and one measurement can mislead.

Compare the result with the new lug height for that size and pattern, which the manufacturer publishes in its data book. As a guide, a new R-1 drive tyre in a common size such as 18.4-38 has lugs of around 40–45 mm; R-1W patterns are about 20 % deeper; row-crop and implement tyres are shallower.

Record the figures with the date and the hour-meter reading. Two readings a year apart tell you the wear rate, and the wear rate tells you when the tyre will need replacing — which is far better known in January than in the middle of drilling.

What the numbers mean in the field

Wheel slip on a tractor doing heavy draft work is normally in the 8–15 % range on a good tyre. As lug height falls, slip climbs, and every percentage point of slip is fuel burned without moving the implement.

• Down to about 50 % of new lug height, the tyre still works well in dry and moderate conditions but slip increases noticeably on wet soil.
• Between 50 % and 25 %, expect slip to rise several points across the board, with more wheel spin at headlands and on slopes. This is the band where most of the hidden cost is paid.
• Below 20–25 % remaining, the tyre has effectively lost its traction pattern. It may still be usable on a trailer or a yard tractor, but not on a draft tractor.

Wear patterns and what they tell you

• Faster wear on one side of the lug points to the tractor spending most of its life pulling in one direction under heavy load — normal on a draft tractor, but check toe on the front axle.
• Faster wear at the shoulder usually means too much pressure for field work.
• Faster wear at the centre means too little pressure or too much road work at low pressure.
• One tyre wearing faster than its partner on the same axle points to a pressure difference or a mismatched pair; equalise or replace both.

When replacement pays

Put a number on it. A 150-hp tractor burning 25 l/h in cultivation with slip 5 points higher than it should be wastes roughly one litre in twenty. Over a 500-hour season that is around 600 litres of diesel — before counting the extra hours, the field days lost to a stuck tractor and the compaction from spinning wheels.

For most farms the right moment is when the lugs reach 25–30 % of new height: plan the replacement before the season, not during it, so the tyres arrive when the tractor is idle. Replacing as a pair on the driven axle keeps the differential and the four-wheel-drive system working as designed.

Send us the tyre size, the tractor model and the current lug height and we will tell you what fits, what is in stock and what the alternatives cost.