In the market for new tractor tires? This guide is a quick primer to give you all of the information you’ll need to find properly fitted tires for the job, including how tires are sized and designated.
In the U.S., tractor tires are regulated by the Department of Transportation and are given an ‘agricultural’ tire classification. This classification encompasses not only tractors, but combines, harvesters, sprayers, trailers and other heavy agricultural equipment, as well. These tires are notable for having very prominent, strategically-placed lugs in order to grip the terrain they are traditionally used on. Tire size is always indicated on the tire wall facing you.
As with any vehicle, your tractor’s rims are a direct determinant of the diameter of the tire you’ll be restricted to. This is why they should always match the rims’ size. More pressure will be required for tires that do not have a width as wide as your rims’ width, or the tractor will suffer from poor traction.
Tractor tires are always size-classified by two particular sizing attributes: “width” and “rim diameter.” Sometimes, you’ll see them advertised with three numbers, which would signify “height,” “width” and “rim diameter.”
For example, let’s say you’re looking at tires that are advertised as “18.4-38”
- Width: 18.4″ wide
- Dash: This dash means it’s a “bias” constructed tire
- Rim Diameter: Fits on a 38″ rim
In other cases, three numbers are used to advertise a tire, such as “27/13.6R38”. Here’s how you would read this number:
- % Height: 27 high
- Width: 13.6″ wide
- R: The “R” means it’s a “radial” constructed tire
- Rim Diameter: Fits on a 38″ rim
Recently, sizing in terms of millimeters (mm) has become more common, and you may come across tractor tires advertised not in inches but millimeters, with a slew of other numbers that give detailed information about the tire. In this case, the syntax would be advertised as:
Width (in mm)/ Aspect Ratio (in %) R (…which stands for “radial”) or B (…which stands for “bias”) Rim Diameter, Load Index, Speed Symbol, Tread Designation
In neater, more illustrative terms — when advertised, the tire would be advertised like this:
432/80R32 140 A7 R1
All tractor tires are constructed in one of two ways: bias or radial. As mentioned above, they are designated with either a dash “-” for bias, or a capital “R” for radial, as seen in the sizing information of the tire. Here are the main differences between the two:
- Bias: These tires are typically the lesser expensive of the two, and are favored for utility usage with overall low hour usage per year, and little time spent on the road.
- Radial: These tires are of a heavier construction, are favored for tillage & field work (especially wet turf or mud), have more traction and fare better on pavement than bias tires.
You may come across tires that have a 1-3 star rating, which designates the maximum amount of tire pressure it has, measured in PSI. The stars typically appear after the stated width and diamteter of the tire (for instance, 11.2-38*** means a 3-star rating). The ratings dictate the following:
- 1-star: 18 psi maximum
- 2-stars: 24 psi maximum
- 3-stars: 30 psi maximum
Aside from size, tractor tires are also manufactured and classified by tread designation. The common designations are:
- R1: Also known as “bar tread,” this is the most common run-of-the-mill tread type for agriculture in North America. Designed for use on farms that prominently have dry earth.
- R1W: Featuring treads that are approximately 25% deeper than an R1, it is designed for moist or wet soil and is more commonly seen for usage in Europe.
- R2: These treads are even deeper than R1W; actually, the deepest of all tire tread designations. They’re made for extremely wet conditions and are rarely used.
- R3: Designed for minimal soil damage, this tread type is better for mowing or other lawn care activities, as it has half the tread depth of an R1, and suffers in traction.
- R4: Designed for industrial use, these tires have about 70% of the depth of an R1. In essence, they have less traction than an R1, but more than an R3.
- F2: For front tires, generally used on a 2WD tractor with R1 or R1W rear tires. These are steering tires that typically have 2 or 3 ribbed grooves.
- F3: Also front tires primarily for 2WD tractors, this designation features multiple ribbed grooves with a varying pattern.
- I3: Favored for industrial use, and features a directional tread pattern with deep grooves.
Tractor tires also list the maximum speed allowed for the stated load of that tire. The speeds are classified as follows:
- A1: 2.5 MPH
- A2: 5 MPH
- A3: 10 MPH
- A4: 12.5 MPH
- A5: 15 MPH
- A6: 20 MPH
- A7: 22.5 MPH
- A8: 25 MPH
- B: 30 MPH
- C: 35 MPH
- D: 40 MPH
- E: 43 MPH
- F: 50 MPH
- G: 55 MPH
Tires With or Without Tubes?
One of the final considerations is to purchase tires that include or do not include tire tubes. Tires without tubes last longer, as they are far more easy to repair and do not require you to remove the tire, then remove the tube for repair work. Tubes are generally a requirement for tires that require liquid ballast, which is inserted into the tube itself. The choice for tubed versus no-tube tires ultimately weighs on this requirement, or purely on the tractor operator’s preference.
Tires for Antique Tractors
Many antique tractors will not have the same tire numbering system that is currently used. From 1932-1940, rubber tires were still a new concept, and the numbering system was based on the widely used 8″ wide rear wheel. In 1938, wider rims were discovered to increase performance and the numbering system changed once again from 1938 to 1960. The current numbering system was adopted in 1955 and is still used to the present day.
Here’s a list of old rim sizes (1938-1960) vs. modern-day width tire sizes that can be used as a rule of thumb:
- Old 8″ rim = Today’s 8.3″ wide tire
- 9″ = 9.5″
- 10″ = 11.2″
- 11″ = 12.4″
- 12″ = 13.6″
- 13″ = 14.9″
- 14″ = 15.5″
- 15″ = 16.9″
- 16″ = 18.4″
- 18″ = 23.1″
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