An agricultural tire sidewall carries three separate pieces of information and people usually read only one. The size tells you the dimensions: in 18.4R42, per AGTireTalk's sidewall identification reference read September 7, 2026, "18.4 = Section Width (inches)", "R = Radial" and "42 = Rim Diameter (inches)", while a dash in place of the R means bias construction. In a metric size such as 480/80R42, "480 = Section Width in Millimeters", "80 = Aspect Ratio (% of sidewall height to section width)" and "42 = Rim Diameter (inches)". The load index or ply rating tells you what it can carry and at what pressure. The R classification tells you nothing about size at all: it describes the tread.
The number that decides whether a different size can go on your machine is none of those. On a machine with mechanical front wheel drive it is rolling circumference, and it is the reason a tire swap that measures correctly at the rim can still be wrong.
The size, field by field
Two conventions are in use on the same yard, which is where most confusion starts.
Standard, or inch, sizing. 18.4R42 or 18.4-42. The first figure is the section width in inches, the letter or dash is the construction, and the last figure is the rim diameter in inches. Nothing in this format states the sidewall height, which is why two tires with the same first and last numbers are not necessarily the same overall diameter.
Metric sizing. 480/80R42. The first figure is the section width in millimeters, the second is the aspect ratio as a percentage of section width, the letter is the construction and the last is still the rim diameter in inches. The aspect ratio is the field the inch format does not have, and it is the one that lets you work out the sidewall height.
Both formats describe the tire. Neither describes the wheel it must be mounted on beyond the rim diameter, and rim width is its own specification from the wheel and tire manufacturers.
Load index, ply rating and star markings
These are three ways of saying what the tire may carry, and they are not interchangeable.
The AGTireTalk reference, read September 7, 2026, defines ply rating as a marking that "Designates maximum load carrying capacity of a specific size tire at maximum air pressure and speed", used mainly on bias tires, and load index as one that "Designates maximum load carrying capacity of a tire at maximum air pressure and speed, in accordance with Load Index Chart". The star markings are a pressure notation on some agricultural tires, listed on the same page as "*=18 PSI, **=24 PSI, ***= 30 PSI". A speed symbol appears alongside the load index and "Designates maximum speed of tire in accordance with Alphabetical Speed Rating Chart".
What a load index means in weight is read from a chart rather than calculated. To take a manufacturer example rather than a general claim, Dave Paulk of BKT USA, quoted in AGTireTalk's load and inflation guidance published June 2, 2025 and read September 7, 2026, states that "The 159 Load Index means that this tire will carry 9,640 per tire running single at 30 mph." That figure belongs to that index at that speed, and it is an illustration of how the chart works rather than a number to apply to your tire.
What R1, R1W, R2, R3 and R4 actually change
The R classification is the tread, and it changes traction, ground disturbance, wear on hard surfaces and ride. It is not part of the size and it does not by itself decide fitment.
| Classification | What the manufacturer describes | Where it is aimed |
|---|---|---|
| R-1 | Agricultural tires with "directional, widely spaced lugs designed for optimal traction in challenging terrain and soft surfaces" | General field work |
| R-1W | "at least a 20% deeper wear pattern" than R-1, and "can be used both on the field and on the road" | Wetter, stickier soils |
| R-2 | Tread depth "can be even double that of R-1 tires", and "used only under extremely wet conditions" | Flooded and very wet ground |
| R-3 | "Turf tires, are designed to minimize damage to the ground. They have shallow treads and closely spaced lugs" | Ground you do not want marked |
| R-4 | "Industrial tires, are the most versatile type of tractor tires, suitable for both agricultural and contracting work" | Hard surfaces, mixed duty |
Every quoted description above is from Nokian Tyres' "Tread types: R-1, R-2, R-3, and R-4 tractor tires", dated 18.08.2025 and read September 7, 2026. The same page carries two cautions that are more useful than most buying advice: R-2 should not be chosen "for general agricultural or contracting operations", and R-3 turf tires should be used "in dry weather conditions, as the shallow and closely spaced treads can quickly clog, causing a loss of traction". It also states that R-4 industrial tires "excel on hard surfaces such as gravel and concrete but should not be used in muddy conditions, unlike R-1 and R-1W tires."
We publish no lug depth in millimeters, no traction percentage and no wear life figure for any of these classifications, because those vary by manufacturer and by pattern within a classification, and the numbers that exist belong to specific tires rather than to the letter on the sidewall.
The check that actually decides a size change
Here is the part that the sidewall does not tell you and that most tire size explainers leave out.
On a machine with mechanical front wheel drive, the front and rear tires are a matched pair. The front tires are smaller and must turn faster, and the relationship between the two is expressed as lead, or lead and lag. AGTireTalk's lead and lag reference, read September 7, 2026, records Chris Neidert of Trelleborg describing lead and lag as "a condition that exists on MFWD (Mechanical Front Wheel Drive) tractors", and defines rolling circumference, in Trelleborg's words dated January 30, 2023, as the "Distance covered in one complete revolution of the wheel on an asphalt road".
The acceptable window is narrow and the manufacturers quoted on that page state it in similar terms. Ken Brodbeck of Precision Inflation: "Front Tires should turn 1 to 5% faster than the Rear Tires". Dave Paulk of BKT USA: a "positive Front Lead (or slippage) of 1% to 5%". Greg Jones of Firestone Ag: "ideal Lead/Lag is 2%, but 0-5% is acceptable". Blaine Cox of Yokohama: "appropriate lead ratio should be 0 to 5%, ideally 3%". The same page records Michelin's statement that "Most tractor manufacturers impose a mechanical lead of between 0% and 6%."
The consequences of getting outside that window are also on the record. Michelin, quoted on the same page, warns that "Improper Lead/Lag can cause more rapid tire wear, increased fuel consumption, transmission unit wear, loss in power and performance, and deterioration of topsoil."
Read that list again, because "transmission unit wear" is the item that turns a tire decision into a machine decision. Changing one end of a mechanical front wheel drive machine without checking the ratio against the other end is not a tire question, it is a driveline question, and the ratio your machine requires is published by the machine's manufacturer for your model. If you do not know which specification applies to your machine, our page on where the model and serial number are stamped covers where those numbers live and why the serial number decides which document applies.
Inflation is set from a table, not from a feel
Agricultural tire pressure is a function of the load carried and the speed traveled, and it is read from the tire manufacturer's own load and inflation table.
The guidance from the manufacturers quoted in AGTireTalk's load and inflation reference, published June 2, 2025 and read September 7, 2026, is consistent. Chris Neidert of Trelleborg and Mitas describes the method as determining "how much air pressure is needed to carry a certain load at a certain speed". James Crouch of Yokohama says that on equipment without central tire inflation you "calculate the inflation pressure for the worst-case scenario. That is, the fastest speed it will travel and the heaviest load." Greg Gilland of Maxam notes that "Depending on the amount of weight and the desired speed, each tire will need to have their cold inflation adjusted."
Two practical consequences. Pressure that was correct with a light implement is not automatically correct with a heavy one, and pressure set for field speed is not automatically correct for a road move between fields. And "cold inflation" is part of the specification, not a detail, because a working tire is warm and reads higher.
We publish no pressure figure on this page. The table for your tire, in your size, at your load and speed, is the tire manufacturer's document, and AGTireTalk's own standing note on that reference is the correct one to carry: "All deductions made from information on this site must be confirmed by Certified Ag Tire Dealer & Tire Manufacturer before use."
Ballast belongs to the same conversation
Liquid ballast in a tire and wheel weights both change the load the tire is carrying, which changes the pressure that load and inflation table calls for. They also change axle loading and machine balance. Whether a given tire may be ballasted with liquid, with what, and to what fill level, is a statement from the tire and wheel manufacturers and from the machine manufacturer, not a general practice. Adding ballast without revisiting the inflation figure moves the tire outside the condition the table assumed.
Where you stop and confirm
Verify compatibility with the equipment manufacturer, the owner's manual, or a qualified equipment professional before installation or operation. On tires there is a second authority as well, because the tire manufacturer and a certified agricultural tire dealer own the load, inflation and fitment answers for the tire itself.
Stop and get that confirmation, rather than buying or fitting, if any of the following is true:
- The machine has mechanical front wheel drive and you are changing the size at one end only, or you cannot confirm the resulting lead against the machine manufacturer's published requirement.
- The rim width or wheel is not confirmed as approved for the tire size you are considering.
- The load index or ply rating of the candidate tire is lower than what the machine and its heaviest work require, or you cannot find the load and inflation table for that tire.
- You are considering an R classification for conditions the tire manufacturer states it is not for, such as turf tires in mud.
- Liquid ballast is in the tire, or is being considered, and the fill and pressure have not been confirmed for that tire and wheel.
Mounting, demounting and inflating an agricultural tire is specialist work with a serious injury history, and it belongs to a tire service technician with the correct equipment. This page does not describe how to do it, how to seat a bead, or how to work on a wheel assembly, and neither does it describe any procedure involving a wheel that is still under load.
The honest limits of this page
We did not measure a tire, fit a wheel or run a machine to write this, and this site does not test equipment. Every quoted definition above is attributed to the manufacturer or reference that published it, on the date it was read. We deliberately publish no load table, no pressure, no lug depth and no rolling circumference figure, because those are per tire and per machine numbers held by the tire manufacturer and the machine manufacturer.
We also could not source a single authoritative document that defines the R classifications in dimensional terms for all makers, which is the honest reason this page describes them in the manufacturers' own words rather than with a table of depths. Where a specific tire's tread depth matters to your decision, it is published for that tire, by its maker. The wider sequence this check sits inside, from make and model through to application, is set out in our overview of how farm equipment fitment is established.
Frequently asked questions
What does 18.4R42 mean on an agricultural tire?
Section width of 18.4 inches, radial construction, and a 42 inch rim diameter, per AGTireTalk's sidewall identification reference. A dash in place of the R indicates bias construction. The format does not state sidewall height, which is why a metric size such as 480/80R42 carries an aspect ratio and this one does not.
Can I put R4 tires on a machine that came with R1?
It is a size question and a duty question at the same time. The R classification describes the tread, so the size, load index, rim approval and, on mechanical front wheel drive machines, the rolling circumference relationship with the other axle all still have to be checked. Nokian Tyres describes R-4 industrial tires as versatile on hard surfaces but not for muddy conditions, so the change also changes what the machine is good at.
What is the difference between R1 and R1W?
Nokian Tyres states that R-1W tires have "at least a 20% deeper wear pattern" than R-1 and can be used on the field and on the road. Deeper tread aims at wetter, stickier soils.
Why does changing only my front tires cause a problem?
Because on a mechanical front wheel drive machine the front tires are meant to turn a small percentage faster than the rears, and changing one end changes that percentage. The manufacturers quoted above place the acceptable band roughly between 0% and 6%, and Michelin lists rapid tire wear, higher fuel use and transmission unit wear among the consequences of getting it wrong.
What pressure should I run?
Whatever the tire manufacturer's load and inflation table gives for your tire size, at the heaviest load and the fastest speed you will use, measured cold. It is not a preference, and it changes when the implement or the job changes. Confirm it with the tire manufacturer or a certified agricultural tire dealer.