How to read tractor tyre markings
The sidewall tells you most of what you need: size, construction, load capacity and permitted speed. This guide goes through the markings using a typical tractor tyre: 650/65R42 158D.
The size: 650/65R42
| Part | Meaning | Example |
|---|---|---|
| 650 | Nominal section width in mm | 650 mm |
| 65 | Aspect ratio: sidewall height as % of width | 650 × 0.65 ≈ 423 mm |
| R | Radial (bias tyres use a hyphen) | Radial |
| 42 | Rim diameter in inches | 42 × 25.4 = 1,067 mm |
Approximate diameter
Diameter ≈ rim diameter (mm) + 2 × width × aspect ratio/100
1,067 + 2 × 423 ≈ 1,912 mm. Michelin's databook gives 1,931 mm for the Multibib 650/65R42. The calculation is a good estimate, but the databook figure applies, especially for front axle lead, which uses rolling circumference.
Inch sizes and size conversion
Older and some current tyres use inch sizes such as 16.9R38: about 16.9 inches wide (about 429 mm) on a 38-inch rim. No aspect ratio is given, but it corresponds to about 85 %, which is why 16.9R38 is commonly matched with 420/85R38. Bias tyres use a hyphen, for example 18.4-38. The catalogue on Däcktryck.se shows the size equivalent where one exists.
Load index and speed symbol: 158D
158 is the load index (LI), a code for the maximum load per tyre at the speed given by the symbol. D is the speed symbol, 65 km/h. LI 158 means 4,250 kg, which matches Michelin's table for the 650/65R42 Multibib: 4,250 kg at 65 km/h and 1.6 bar.
Load index (selection)
| LI | kg | LI | kg | LI | kg |
|---|---|---|---|---|---|
| 140 | 2,500 | 152 | 3,550 | 164 | 5,000 |
| 142 | 2,650 | 154 | 3,750 | 166 | 5,300 |
| 144 | 2,800 | 155 | 3,875 | 168 | 5,600 |
| 146 | 3,000 | 156 | 4,000 | 170 | 6,000 |
| 147 | 3,075 | 158 | 4,250 | 173 | 6,500 |
| 148 | 3,150 | 160 | 4,500 | 176 | 7,100 |
| 150 | 3,350 | 161 | 4,625 | 178 | 7,500 |
| 151 | 3,450 | 162 | 4,750 | 181 | 8,250 |
Speed symbols
| Symbol | A2 | A6 | A8 | B | C | D | E | F |
|---|---|---|---|---|---|---|---|---|
| km/h | 10 | 30 | 40 | 50 | 60 | 65 | 70 | 80 |
Dual markings
Many tyres carry two values, for example 165D/161E (Michelin RoadBib 650/65R42): 5,150 kg at 65 km/h or 4,625 kg at 70 km/h. Others, such as 154A8/154B, mean the same load, 3,750 kg, at both 40 and 50 km/h.
The load index applies at the tyre's reference pressure. At other pressures and speeds the manufacturer's load and pressure table applies. See tractor tyre pressure.
IF and VF
IF (Improved Flexion) and VF (Very High Flexion) tyres have sidewalls that tolerate more deflection. Under the industry standard, an IF tyre can carry up to 20 % more load than a standard tyre of the same size at the same pressure, and a VF tyre up to 40 %. Alternatively they can carry the same load at lower pressure, which reduces soil compaction. The marking comes before the size, for example VF 710/70R42. Each IF/VF tyre has its own table, so always use it.
Other markings
| Marking | Meaning |
|---|---|
| TL / TT | Tubeless / tube type |
| CFO | Cyclic Field Operation – higher load under cyclic loading, for example combines, under the manufacturer's conditions |
| R-1 | Standard agricultural tread |
| R-1W | About 20 % deeper tread than R-1, for grip and tyre life |
| R-2 | Very deep tread for wet conditions such as rice and sugar cane |
| Production date | Often four digits for week and year, e.g. 2324 = week 23 of 2024 |
Common mistakes
- Treating the sidewall pressure as the working pressure. It is normally the maximum.
- Comparing load indices of tyres with different speed symbols without converting.
- Assuming two tyres of the same size are the same size. See rolling circumference.
Same size, different load index
| 650/65R42 tyre | Marking | Load at marked speed |
|---|---|---|
| Michelin Multibib | 158D | 4,250 kg at 65 km/h |
| Michelin RoadBib | 165D/161E | 5,150 kg at 65 or 4,625 kg at 70 km/h |
| Nokian Ground King | 170D/167E | 6,000 kg at 65 or 5,450 kg at 70 km/h |
| Vredestein Traxion Optimall VF | 174D | 6,700 kg at 65 km/h |
A higher load index means more capacity, but usually at a higher reference pressure. At low pressures the table shows the real difference.
About the tables
The load index and speed symbol tables above were checked against Bridgestone's and Firestone's published agricultural tyre tables. More speed symbols exist, for example A1 (5), A3 (15), A4 (20), A5 (25), A7 (35) and G (90 km/h).
Suffixes after the size
Some sizes carry manufacturer suffixes such as CFO and CHO (cyclic loads) or IND (industrial use). Meanings and conditions are given in each maker's databook, so read them before relying on extra capacity.
Related questions
What does A8 mean on a tractor tyre?
Speed symbol A8 means 40 km/h. A tyre marked 154A8 may carry 3,750 kg at up to 40 km/h.
What do the numbers on a tractor tyre mean?
Width in mm, aspect ratio in %, R for radial, rim diameter in inches, then load index and speed symbol, for example 650/65R42 158D.
How do I convert tyre inches to millimetres?
Multiply by 25.4. A 38-inch rim is 965 mm and a 42-inch rim 1,067 mm.
Can I drive faster than the speed symbol?
Not at full load. Some manufacturers give reduced loads for higher speeds in their tables, but without that the symbol is the limit.
Worked example: reading a full VF tyre marking
Trelleborg TM1000 ProgressiveTraction VF 710/70R42 182D/179E:
- VF: Very High Flexion, see IF and VF tyres.
- 710: width 710 mm. 70: sidewall height is 70 % of width, about 497 mm.
- R42: radial, for a 42-inch rim (1,067 mm).
- 182D: 8,500 kg at 65 km/h. 179E: 7,750 kg at 70 km/h.
Approximate outside diameter: 1,067 + 2 × 497 ≈ 2,061 mm. The databook rolling circumference is 6,190 mm, an effective loaded diameter of about 1,970 mm.
Rim diameters in inches and mm
| Inches | 24 | 26 | 28 | 30 | 34 | 38 | 42 | 46 |
|---|---|---|---|---|---|---|---|---|
| mm | 610 | 660 | 711 | 762 | 864 | 965 | 1,067 | 1,168 |
Tyre age: the DOT code
The sidewall often has a code ending in four digits, for example 2324: week 23 of 2024. Manufacturers give different advice on age, so check condition regularly and ask your supplier if in doubt.
Checklist: checking the marking
- Read the size and check it matches the rim.
- Read every load index and speed symbol.
- Check the symbol covers the speed you drive.
- Check the load index covers your load.
- Read the pressure from the maker's table, not just the maximum pressure on the sidewall.
Imperial sizes such as 18.4R38
Older and some current tyres use inch sizes, for example 18.4R38: about 18.4 inches (467 mm) wide, radial, for a 38-inch rim. These roughly correspond to the metric 85 series, and 18.4R38 is often listed as equivalent to 460/85R38.
How to do it in the app
Search the Catalog by size, for example 650/65R42, or by make and pattern. Each tyre shows load index, dimensions, rolling circumference and size equivalent, and the Pressure guide has the manufacturer's table for over 2,500 tyres.
Open Catalog in the appFrequently asked questions
What does 158D mean on a tractor tyre?
158 is the load index, 4,250 kg per tyre, and D is the speed symbol, 65 km/h.
What does 165D/161E mean?
The tyre may carry 5,150 kg at 65 km/h (165D) or 4,625 kg at 70 km/h (161E).
What is the difference between IF and VF?
Both tolerate more deflection than standard tyres. IF can carry up to 20 % more load at the same pressure and VF up to 40 %, or the same load at lower pressure.
What is 16.9R38 in metric?
Roughly 420/85R38: about 429 mm wide with about 85 % aspect ratio on a 38-inch rim.
What does A8 mean on a tractor tyre?
A8 is the speed symbol for 40 km/h.
Last updated 2026-10-15. General information. See disclaimer.