Lawn Tractor and Mower Tire Sizes: Markings, Charts and Dimension Comparisons

What do the numbers on a lawn tractor tire mean?

Most lawn tractor and garden tractor tires use a three-number tire size such as 20x8.00-8. Read it as overall diameter x section width - rim diameter, with all three values in inches. A 20x8.00-8 tire is nominally 20 inches tall, about 8 inches wide, and mounts on an 8-inch diameter rim.

The size code describes the tire's nominal dimensions. Other markings on the sidewall describe its strength, load capacity, speed limit, construction and recommended pressure. These ratings are not dimensions and should be read separately.

How to read three-number lawn tractor tire sizes

A three-number size lists the overall diameter first, the section width second and the rim diameter last. The dash or hyphen comes right before the rim diameter. The format is used for most riding mower, lawn tractor, garden tractor and zero-turn mower tires.

Overall diameter (first number)

The first number is the nominal height of the inflated tire in inches, from the bottom of the tread to the top. In 20x8.00-8, the overall diameter is 20 inches. This number determines axle height, ground clearance and how far the tire travels in one turn.

Section width (second number)

The second number is the nominal width of the inflated tire in inches, measured across the widest point of the sidewalls. In 20x8.00-8, the section width is 8 inches. Writing it as 8.00 or 8 means the same nominal width. Section width is measured on a specified rim width, so the same tire can measure slightly differently on a narrower or wider rim.

Rim diameter (third number)

The third number is the diameter in inches of the rim the tire bead sits on. In 20x8.00-8, the rim diameter is 8 inches. The tire's rim diameter must match the wheel's rim diameter exactly. An 8-inch tire cannot be mounted on a 6-inch or 10-inch wheel.

Steps to read a lawn tractor tire size

1. Find the size code on the sidewall. It is usually the largest group of numbers, for example 18x9.50-8 or 23x10.50-12.
2. Take the number before the x as the overall diameter in inches.
3. Take the number between the x and the dash as the section width in inches.
4. Take the number after the dash as the rim diameter in inches.
5. Read the other sidewall markings, such as ply rating, load index, maximum pressure and tube type, separately. They describe performance and construction, not size.

Other lawn mower tire size formats

Not every lawn mower tire uses the three-number format. Small riding mower front tires, utility wheels and some imported tires use two-number or metric codes, and many walk-behind mower wheels are sold by wheel diameter only.

Two-number sizes such as 4.10/3.50-4

A two-number size gives the section width and the rim diameter, but not the overall diameter. In 4.80/4.00-8, the figures before the dash are section-width designations in inches, and 8 is the rim diameter in inches. The pair 4.80/4.00 shows that the tire covers two interchangeable size designations, one newer and one older. Because the overall diameter is not part of the code, you need the manufacturer's published specification to compare its height with another tire.

Metric sizes with an aspect ratio such as 145/70-6

A metric-style size gives the section width in millimetres, the aspect ratio as a percentage and the rim diameter in inches. In 145/70-6, the tire is nominally 145 mm wide. The sidewall height is 70 percent of that width, and the tire fits a 6-inch rim. The overall diameter has to be calculated, as shown later in this article.

Walk-behind lawn mower wheel sizes

Push mower and self-propelled mower wheels are usually described by overall wheel diameter in inches, such as 7-inch, 8-inch or 11-inch wheels. Many of these are molded plastic or semi-pneumatic units, not separate tires on rims. For these wheels, the overall diameter, the axle or bore size and the mounting style matter more than a sidewall size code.

Sidewall markings that are not dimensions

Lawn tractor tires often carry ratings and construction markings next to the size code. These describe what the tire can carry and how it is built, not how big it is.

• Ply rating (PR) or load range: a marking such as 4PR is an index of carcass strength and load capacity. It does not necessarily mean the tire contains that number of physical plies. A higher ply rating generally indicates a stiffer, higher-capacity tire of the same size.
• Load index and speed symbol: a service description such as 74A4 combines a load index and a speed symbol. Load index 74 corresponds to 375 kg per tire under the specified conditions. Speed symbol A4 corresponds to 20 km/h. These ratings let you compare carrying capacity and speed limits between tires of the same size.
• Maximum inflation pressure: usually given in PSI and sometimes also in kPa. This is the highest cold pressure the tire is designed for. It is not the recommended operating pressure for your machine, which is set by the equipment maker.
• Maximum load: sometimes printed in pounds and kilograms, usually tied to the stated pressure.
• Tubeless or Tube Type: this shows whether the tire is designed to seal on the rim or needs an inner tube.
• NHS (Not for Highway Service): this indicates the tire is intended for off-road equipment use, not for public road vehicles.
• Tread type and rotation arrow: names such as turf, lug, bar, ribbed or chevron describe the tread pattern. A rotation arrow shows the intended rolling direction for directional treads.

How width, aspect ratio and rim diameter determine tire geometry

Three values determine a tire's basic geometry: section width, sidewall height and rim diameter. Aspect ratio links sidewall height to width. Sidewall height and rim diameter together set the overall diameter, and the overall diameter sets the circumference.

Formulas for three-number sizes (inches)

• Sidewall height = (overall diameter - rim diameter) / 2
• Approximate aspect ratio (%) = sidewall height / section width x 100
• Circumference = overall diameter x 3.1416

Example, 20x8.00-8: sidewall height = (20 - 8) / 2 = 6 inches. Aspect ratio is about 6 / 8 x 100 = 75 percent. Circumference = 20 x 3.1416, or about 62.8 inches, so the tire travels roughly 62.8 inches per full turn.

Formulas for metric sizes (millimetres and inches)

• Sidewall height (mm) = section width in mm x aspect ratio / 100
• Sidewall height (in) = sidewall height in mm / 25.4
• Overall diameter (in) = rim diameter + 2 x sidewall height in inches

Example, 145/70-6: sidewall height = 145 x 0.70 = 101.5 mm, or about 4.0 inches. Overall diameter = 6 + 2 x 4.0, or about 14.0 inches. Circumference is about 14.0 x 3.1416, or 44 inches. Section width is 145 / 25.4, or about 5.7 inches.

Lawn tractor tire size chart with calculated dimensions

The tire size chart below lists common lawn tractor, garden tractor, riding mower and zero-turn tire sizes. Sidewall height, aspect ratio and circumference are calculated from the nominal size code. They are not measured values for any specific brand or tread. The position column shows where each size is commonly seen, but the correct size for a given machine comes from its manufacturer.

Some sizes share the same overall diameter and rim diameter but differ in width, such as 20x8.00-8 and 20x10.00-8. On paper they have the same height and circumference. The wider tire has a lower aspect ratio and a wider footprint.

Lawn tractor tire size comparison: what changes when one number changes

To compare two lawn tractor tire sizes, look at each number separately. A change in overall diameter affects height, ground clearance and travel per revolution. A change in section width affects footprint and space needed beside the tire. A change in rim diameter requires a different wheel.

Changing overall diameter

A larger overall diameter raises the axle and increases the distance covered per turn. The percentage difference is (new diameter - old diameter) / old diameter x 100.

Example, 18x8.50-8 vs 20x8.00-8:

• Overall diameter rises from 18 to 20 inches, a difference of 2 inches, or about 11.1 percent.
• The axle centre rises by about half the diameter difference, roughly 1 inch.
• Circumference rises from about 56.5 to 62.8 inches, also about 11.1 percent more travel per turn.
• At the same axle speed, ground speed rises in the same proportion, so the machine covers about 11 percent more ground per wheel revolution.
• On a lawn tractor, raising only the rear axle tilts the frame, and with it the mower deck. That changes cutting height and front-to-rear deck levelness.

Changing section width

With the same overall diameter and rim diameter, a wider tire mainly changes width and footprint, not height. Comparing 20x8.00-8 with 20x10.00-8, the nominal overall diameter (20 inches), sidewall height (6 inches) and circumference (about 62.8 inches) are the same. Only the section width grows by 2 inches. The wider tire can spread the load over a wider contact area. It also needs more side clearance and a rim width approved for that tire.

Changing rim diameter

For the same overall diameter, a larger rim diameter leaves less room for sidewall. For illustration, a 20-inch-tall tire on an 8-inch rim has a sidewall height of (20 - 8) / 2 = 6 inches. A 20-inch-tall tire on a 12-inch rim would have only (20 - 12) / 2 = 4 inches. A shorter sidewall usually means less cushioning and a lower aspect ratio. A different rim diameter always means a different wheel.

Comparison table for common rear tire sizes

Front-to-rear diameter on four-wheel-drive garden tractors

On a garden tractor with a mechanically driven front axle, the front and rear tire diameters work together with the gearing. Changing the overall diameter on only one axle alters the relationship between front and rear ground speed. That can cause driveline strain or tire scrubbing. On these machines, front and rear sizes should be changed only in combinations the manufacturer approves.

Calculated tire size vs actual tire dimensions

The numbers in a tire size code are nominal design dimensions, not exact measurements. Two tires marked 20x8.00-8 from different manufacturers, or with different tread patterns, can measure differently in height and width. Calculations from the size code are useful for comparing sizes on paper, but they do not tell you the exact physical size of a particular tire.

Actual lawn tractor tire dimensions depend on:

• Brand and internal construction
• Tread pattern and tread depth
• The width of the rim the tire is mounted on
• Inflation pressure
• The load on the tire, which flattens the bottom and reduces the loaded radius
• Wear, which reduces overall diameter over time

The manufacturer's published specifications for a specific tire list its measured overall diameter and section width on a stated rim. Use those values when a small difference matters. You can also measure a tire yourself:

1. Mount the tire on its rim and inflate it to the pressure recommended for the machine.
2. With the tire unloaded, measure the overall diameter across the centre of the tread from top to bottom.
3. Measure the section width at the widest point of the sidewalls, ignoring raised lettering.
4. For loaded height, measure from the ground to the axle centre with the machine on a level surface, and compare that with half the unloaded diameter.

Tire size match vs fitment compatibility

Matching size numbers shows that two tires are nominally the same size. It does not prove that a tire or wheel will fit and work on a particular lawn tractor, garden tractor or mower. Fitment depends on the whole wheel and machine, not just the tire code.

Before treating two tires or wheels as interchangeable, check:

• Rim diameter: it must be identical.
• Rim width: it must suit the tire's section width.
• Wheel mounting: hub style, bolt pattern or axle bore, and offset.
• Clearance: space between the tire and the fenders, frame, mower deck and steering parts, through the full steering range.
• Load and ply rating: these should be suitable for the machine's weight and any attachments.
• Construction: whether the wheel needs a tube-type or tubeless tire.
• Drive effects: effects on ground speed, deck height, levelness and, on front-drive models, front-to-rear tire ratio.

The size code is the right starting point. Width, aspect ratio and rim diameter explain how one tire's height, sidewall and circumference compare with another's. Actual measurements and the equipment maker's approved sizes decide whether a specific tire belongs on a specific machine.