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Tractor Tire Load Index & Speed Rating Chart
A farm tire's load index and speed symbol are the code after the size, such as 155A8. The number is the load index: the most that one tire can carry, read from a standard chart. 155 means 3,875 kg (8,543 lbs). The letter is the speed symbol: the top speed for that load. A8 means 40 km/h (25 mph). The load index applies at the tire's maximum rated pressure.
Farm tires use the slow end of the international speed scale. A1 to A8 cover 5 to 40 km/h, then B is 50 km/h, C 60, D 65, E 70, F 80 and G 90. Car tires use letters from the top of the same scale, such as S (180 km/h) or V (240 km/h) — a car-tire chart won't help with a tractor tire.
Many radial farm tires carry two ratings, like 163D/166A8 — a separate load rating for each of two speeds. Look up any load index from 40 to 200 or any farm speed symbol below. Kilograms and km/h are the standard values from tire makers' data books; pounds and mph are converted. To set tire pressure, use the tire maker's load and inflation table for your specific tire, not this chart.
How to read a load index and speed rating
\nFind the code after the size on your sidewall, then look it up in the load index and speed rating chart above. To set pressure for the load you carry, follow the tire maker table and our tractor tire pressure guide. For tires that carry more at lower pressure, see IF vs VF tires.
Questions
What does the A8 in a tractor tire size mean?
A8 is the speed symbol. It means the tire carries its rated load at up to 40 km/h (25 mph). The number in front of it is the load index: in 155A8, 155 means 3,875 kg (8,543 lbs) per tire.
What do two ratings like 163D/166A8 or 155A8/152B mean?
163D/166A8 means 4,875 kg (10,748 lbs) at up to 65 km/h (40 mph), or 5,300 kg (11,684 lbs) at up to 40 km/h (25 mph). 155A8/152B means 3,875 kg (8,543 lbs) at up to 40 km/h, or 3,550 kg (7,826 lbs) at up to 50 km/h (31 mph). 155A8/155B, sometimes printed 155A8/B, means the same 3,875 kg at either speed.
Is the D in "D/8 ply" a speed rating?
No. On bias farm tires, a letter next to a ply rating — C/6 ply, D/8 ply, E/10 ply — is the load range, a code for casing strength, not a speed symbol. A speed symbol always comes straight after a load index number, as in 131D, where D means 65 km/h (40 mph).
Can I fit a tire with a higher load index than my old one?
Yes. Match or beat the load index and speed symbol of the tires your machine came with — going higher is fine as long as the size fits your rim and machine. Then set pressure for the load the tire actually carries using the tire maker's load and inflation table, not the sidewall maximum. If you also change size on a front-wheel-assist tractor, check the front-to-rear ratio with our lead-lag calculator.
Why does the load table for my tire list different loads at different speeds?
Because speed changes what a tire can safely carry — running faster builds more heat, so the tire needs more air pressure to carry the same load safely. The load index applies at the speed in its speed symbol; a tire maker's load and inflation table lists the load and pressure for each speed, so use the row that matches how fast you actually run, in the field and on the road.