Load index and speed symbol on tractor tyres

Next to the size on a radial tractor tyre you will find a marking such as 146A8 or 149D. The number is the load index and the letter (plus digit) is the speed symbol. Reading both correctly is essential for choosing and using tyres safely.

Load index

The load index is a code for the maximum load a single tyre can carry at the stated speed and recommended pressure. The number is not kilograms; each value maps to a standard table. Higher means more capacity. For example, 140 corresponds to roughly 2,500 kg and 150 to roughly 3,350 kg.

Some tyres show two indices (e.g. 146A8/146B or 149A8/146D), meaning different loads at different speeds: more at low speed, less at high speed.

Speed symbol

Common agricultural speed symbols:

• A2: 10 km/h
• A6: 30 km/h
• A8: 40 km/h
• B: 50 km/h
• D: 65 km/h

The speed symbol is the maximum sustained speed at that load index. The tractor's road speed should not exceed it.

Ply rating (PR)

Bias tyres usually state capacity with a ply rating (e.g. 8 PR, 12 PR) instead of a load index. Today PR is a strength class, not an actual ply count. In the same size, a higher PR carries more load but usually needs more pressure.

When replacing tyres

• The new tyre's load index should not be lower than the factory tyre's.
• The speed symbol must suit the tractor's top road speed.
• With a front loader, front tyre load capacity matters especially.
• Always set pressure from the maker's load–pressure table.

See also sidewall markings and tractor tyre pressure.