Metric Tractor Tire Sizes: How to Read, Convert and Compare Them

A metric tractor tire size such as 480/80R46 gives three things. The section width is 480 millimetres. The aspect ratio is 80, meaning the sidewall height is 80% of that width. The rim diameter is 46 inches. The R means radial construction. Numbers that follow the size, such as 158A8, are performance ratings. 158 is a load index and A8 is a speed symbol. They do not describe dimensions. These three size values let you calculate the tire's nominal sidewall height, overall diameter and circumference. They also let you compare a metric size with an older inch-based size. The calculation is only an approximation, though. It does not replace the manufacturer's published dimensions, and it does not confirm that a tire fits a particular tractor.

How to read a metric tractor tire size on the sidewall

Read a metric tractor tire marking from left to right. It separates into dimensions, a ratio, construction, units and performance ratings. Take a sidewall that reads 480/80R46 158A8 / 155B. Each part of that marking means something different.

Marking	What it is	Unit or type	What it tells you
480	Nominal section width	Millimetres (dimension)	Width of the inflated tire from sidewall to sidewall, measured on its specified rim. Lettering and protective ribs are not included.
80	Aspect ratio	Percent (ratio, no unit of length)	Sidewall height as a percentage of section width: 80% of 480 mm = 384 mm.
R	Construction	Code	Radial carcass. A dash (-) in this position indicates bias (diagonal) construction.
46	Rim diameter	Inches (dimension)	Diameter of the wheel rim the tire mounts on. It must match the rim exactly.
158	Load index	Code (performance rating)	Maximum load per tire at the stated speed and reference conditions. Load index 158 corresponds to 4,250 kg.
A8	Speed symbol	Code (performance rating)	Speed at which the paired load index applies. A8 = 40 km/h.
155B	Second load and speed rating	Code (performance rating)	Many agricultural tires carry two ratings. Here the tire is rated for a lower load (index 155) at a higher speed (B = 50 km/h).

Dimensions, ratios and units are mixed in one designation

The metric size designation mixes units. Section width is given in millimetres and rim diameter in inches. The aspect ratio is a percentage with no unit of length. Before you calculate anything, convert everything to one unit. Multiplying inches by 25.4 gives millimetres.

Performance ratings are not size information

The load index and speed symbol describe how much weight a tire may carry and at what speed. They do not describe how big the tire is. Two tires with the same size can carry different load indexes. Two tires with the same load index can have different sizes. Common agricultural speed symbols are listed below.

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

A load index is always tied to its paired speed symbol and to the inflation pressure stated in the manufacturer's load tables.

Other markings that do not change the size

• IF and VF prefixes (for example IF 710/70R42) mark Increased Flexion and Very high Flexion tires. These tires can carry a given load at lower inflation pressure than a standard tire of the same size. The prefix does not change the meaning of the size numbers.
• Tread codes such as R1, R1W, R2 and R4 classify the tread pattern and tread depth. They are not dimensions. A deeper tread can, however, make the actual overall diameter slightly larger.
• Ply ratings and star markings appear mainly on older inch-sized tires. They indicate load and pressure capacity, not size.

How metric tractor tire sizes differ from inch-based tire sizes

An inch-based tractor tire size such as 18.4R38 gives two numbers. The section width is 18.4 inches and the rim diameter is 38 inches. No aspect ratio is printed. A metric size states width in millimetres and includes the aspect ratio as a separate number.

Traditional inch-sized rear tractor tires have a fairly tall sidewall, and that profile is close to an 85% aspect ratio. That is why so many inch sizes have metric counterparts in the 85 series. The metric system also covers wider, lower-profile sizes in the 80, 75, 70, 65 and lower series. The older two-number inch format has no clean way to describe these profiles.

On inch-based sizes, a dash instead of R indicates bias construction. For example, 18.4-38 is a bias tire and 18.4R38 is a radial.

Metric tractor tire size conversion: common inch-to-metric equivalents

A metric tractor tire size conversion links an inch-based size to the metric size with nearly the same section width, sidewall profile and rim diameter. To approximate it, multiply the inch section width by 25.4 to get millimetres. Then round to the nearest metric width in the 85 series and keep the same rim diameter. For example, 18.4 in × 25.4 = 467 mm, which corresponds to the commonly listed equivalent 460/85R38 for 18.4R38.

Inch-based size	Commonly listed metric equivalent	Rim diameter
11.2R24	280/85R24	24 in
12.4R24	320/85R24	24 in
13.6R28	340/85R28	28 in
14.9R24	380/85R24	24 in
14.9R28	380/85R28	28 in
14.9R30	380/85R30	30 in
16.9R28	420/85R28	28 in
16.9R38	420/85R38	38 in
18.4R34	460/85R34	34 in
18.4R38	460/85R38	38 in
18.4R42	460/85R42	42 in
20.8R38	520/85R38	38 in
20.8R42	520/85R42	42 in
18.4R46	480/80R46	46 in

Treat this size equivalence as a starting point for comparison, not a guarantee. Equivalent sizes are close in nominal dimensions. Actual overall diameter, section width, load capacity and recommended rim width can still differ between the two designations and between manufacturers.

How to calculate sidewall height, overall diameter and circumference

You can calculate the nominal geometry of any metric tractor tire from its three size numbers. Here are the steps, using 480/80R46 as the example.

1. Find the sidewall height. Multiply section width by aspect ratio ÷ 100. 480 mm × 0.80 = 384 mm.
2. Convert the rim diameter to millimetres. Multiply by 25.4. 46 in × 25.4 = 1,168.4 mm.
3. Find the overall diameter. Add the rim diameter and two sidewall heights, since there is one sidewall above and one below the rim. 1,168.4 + (2 × 384) = 1,936.4 mm.
4. Find the circumference. Multiply the overall diameter by π (about 3.1416). 1,936.4 mm × 3.1416 ≈ 6,083 mm, or about 6.08 m.
5. Convert to inches if needed. Divide millimetres by 25.4. 1,936.4 mm ÷ 25.4 ≈ 76.2 in.

The formula in one line: overall diameter (mm) = rim diameter (in) × 25.4 + 2 × section width (mm) × aspect ratio ÷ 100.

Inch-based sizes such as 18.4R38 do not print an aspect ratio. Their geometry cannot be calculated precisely from the marking. Use the manufacturer's published dimensions for those sizes.

Metric tractor tire size chart: calculated dimensions

The metric tractor tire size chart below shows nominal geometry calculated with the formula above. It is useful for comparing sizes with each other. It does not list measured or manufacturer-published dimensions.

Metric size	Section width (mm)	Aspect ratio	Rim (in)	Sidewall height (mm)	Calculated overall diameter (mm / in)	Calculated circumference (m)
320/85R24	320	85	24	272	1,153.6 / 45.4	3.62
380/85R28	380	85	28	323	1,357.2 / 53.4	4.26
420/85R28	420	85	28	357	1,425.2 / 56.1	4.48
540/65R28	540	65	28	351	1,413.2 / 55.6	4.44
600/65R28	600	65	28	390	1,491.2 / 58.7	4.68
460/85R38	460	85	38	391	1,747.2 / 68.8	5.49
520/85R42	520	85	42	442	1,950.8 / 76.8	6.13
650/65R42	650	65	42	422.5	1,911.8 / 75.3	6.01
710/70R42	710	70	42	497	2,060.8 / 81.1	6.47
480/80R46	480	80	46	384	1,936.4 / 76.2	6.08

How width, aspect ratio and rim diameter change tire size

Overall diameter depends on all three size numbers working together. The rim diameter sets the base. Section width multiplied by aspect ratio sets the sidewall height, and the sidewall height counts twice in the overall diameter. Changing any one number changes the outer size, but not always in the direction you might expect.

Wider is not automatically taller

Compare 650/65R42 with 520/85R42. Both use a 42-inch rim. The 650 is 130 mm wider, but its lower 65% aspect ratio gives a sidewall of 422.5 mm against 442 mm. As a result, the wider tire has a calculated overall diameter about 39 mm smaller: 1,911.8 mm vs 1,950.8 mm.

A wider tire with a moderate aspect ratio grows taller

Compare 710/70R42 with 520/85R42. The 710 tire has a 497 mm sidewall against 442 mm. Its calculated overall diameter is 110 mm larger, and its circumference is about 0.35 m larger: 6.47 m vs 6.13 m. That is roughly 5.6% more ground covered per revolution before slip. Over 1,000 m, the 520/85R42 turns about 163 times and the 710/70R42 about 155 times.

A larger rim can offset a lower sidewall

Compare 480/80R46 with 520/85R42. The 480 tire is narrower and has a lower aspect ratio, so its sidewall is 58 mm shorter. Its rim, however, is 4 inches (101.6 mm) larger. The two effects nearly cancel. The calculated overall diameters are 1,936.4 mm and 1,950.8 mm, which differ by less than 1%. This shows how a tall, narrow row-crop tire on a larger rim can end up close in outer size to a wider tire on a smaller rim.

What changes and what stays the same

Change (other two values unchanged)	Sidewall height	Overall diameter	Circumference
Wider section width	Increases	Increases	Increases
Lower aspect ratio	Decreases	Decreases	Decreases
Larger rim diameter	No change	Increases by the rim difference	Increases

Calculated geometry vs actual tire dimensions

The calculated overall diameter of a metric tractor tire is a nominal value. It is not a measurement. Manufacturers publish actual dimensions for each specific tire, and these can differ from the calculation for several reasons.

• Nominal rounding. The numbers in a size designation are rounded design values. The real section width and sidewall height of a given tire may be somewhat larger or smaller.
• Tread depth and pattern. Lug height adds to the outer diameter. Tires in the same size with different tread depths, such as R1 vs R1W, can have different overall diameters.
• Rim width. Section width is defined on a specified measuring rim. Mounting on a narrower or wider permitted rim changes the actual width slightly.
• Load and inflation. Under load the tire deflects. The static loaded radius is smaller than half the unloaded diameter. The rolling circumference is the distance travelled per revolution under working conditions, and it is shorter than the calculated circumference.
• Wear. As lugs wear, overall diameter and rolling circumference decrease.

For any decision that depends on exact dimensions, use the manufacturer's data for that exact tire and tread type. Look for the overall diameter, section width, static loaded radius and rolling circumference.

What dimensional comparisons cannot tell you about fitment

A size comparison shows how two tires differ on paper. It does not show whether a tire is approved or suitable for a particular tractor, wheel or job. Several fitment factors lie outside the size marking.

• Rim diameter must match exactly. A 46-inch tire mounts only on a 46-inch rim. Rim width must also fall within the tire manufacturer's approved range.
• Clearance. A larger overall diameter or section width may contact fenders, cabs, frames or steering components. A wider tire may not fit between crop rows.
• Front-to-rear relationship on four-wheel-drive tractors. On mechanical front-wheel-drive tractors, the rolling circumferences of front and rear tires must stay in the relationship the tractor manufacturer specifies. A change that looks small in the calculation can matter here, so check the manufacturer's approved tire combinations.
• Load and speed capacity. Two equivalent sizes can carry different load indexes and speed symbols. The replacement must meet the axle load and operating speed required.
• Speed indication. A change in rolling circumference changes the ground speed for a given axle speed. It can also affect speed and slip readings that rely on wheel rotation.

Use the size marking and the calculations to understand and compare metric tractor tire sizes. Use the tractor and tire manufacturers' approved fitments and data tables to decide what can actually be mounted.