How To Read Tractor Tire Size | Decode Sidewall Numbers

Tractor tire sidewall codes show width, profile, construction, rim size, and load markings so you can match the right replacement.

Reading a tractor tire sidewall gets easy once you know the order of the numbers and letters. Most farm tires follow a pattern. The first part tells you width, the middle tells you shape or construction, and the last number tells you rim diameter.

That sounds dry on paper, yet it saves real money in the yard. If you misread one number, you can end up with a tire that will not seat on the rim, rub the fender, throw off your front-to-rear rolling match, or carry less weight than your tractor and implement need.

Use this article the same way you’d use a sidewall in front of you. Start with the code, split it into chunks, then match each chunk to its job.

How To Read Tractor Tire Size On The Sidewall

Most tractor tires show size in one of two common styles: an older inch-based format or a newer metric radial format. Once you know which style you’re holding, the rest falls into place fast.

Old Inch-Based Format

An older farm tire may read 12.4-28 or 16.9R30. In that style, the first number is tire width in inches, the dash means bias construction, the letter R means radial construction, and the last number is rim diameter in inches.

Take 12.4-28. You’re reading a tire that is about 12.4 inches wide, built in bias form, and made for a 28-inch rim. Take 16.9R30. That one is about 16.9 inches wide, built in radial form, and made for a 30-inch rim.

Metric Radial Format

A newer sidewall may read 320/85R28, 420/70R24, or 600/65R38. In this style, the first number is width in millimeters, the number after the slash is aspect ratio, the R marks radial build, and the last number is rim diameter in inches.

What The Slash Number Means

The slash number is the sidewall height shown as a share of the tire’s width. So on a 320/85R28 tire, the sidewall height is 85% of 320 mm. A lower ratio, such as 70 or 65, gives a shorter sidewall and a wider, lower-profile shape.

That one figure changes the tire’s outside height more than many owners expect. Two tires can fit the same rim diameter and still stand at different heights because their aspect ratios are not the same.

Reading Tractor Tire Size Codes Without Guesswork

When you’re staring at a dusty sidewall, work left to right. This quick order keeps you from mixing width, construction, and rim size.

• First number: tire width
• Slash number: sidewall ratio on metric sizes
• R or dash: radial or bias build
• Last number: rim diameter in inches

Then check for extra service markings near the size line. Those can include load index, speed symbol, tube type, tubeless marking, ply rating on older bias tires, and brand-specific notes printed elsewhere on the sidewall.

One trap catches a lot of buyers: rim diameter is still stated in inches even on metric tire sizes. A 480/80R42 tire uses a 42-inch rim, not a 42-centimeter rim and not a 480 mm rim.

Sidewall Marking	What It Means	How To Read It Fast
12.4-28	12.4-inch width, bias build, 28-inch rim	Old inch format with a dash
16.9R30	16.9-inch width, radial build, 30-inch rim	Old inch format with R
320/85R28	320 mm width, 85-series, radial, 28-inch rim	Metric radial farm tire
420/70R24	420 mm width, 70-series, radial, 24-inch rim	Lower-profile metric size
600/65R38	600 mm width, 65-series, radial, 38-inch rim	Wide radial rear tire
R	Radial construction	More common on newer field tractors
– or X	Bias construction	Common on older or smaller applications
28, 30, 38, 42	Rim diameter in inches	Always match this number first

What Common Tractor Tire Sizes Tell You

Here’s the part many people miss: tire size tells you fit, shape, and construction, but it does not tell the full working limit by itself. You still need to match the tractor, axle load, speed, and rim width. Firestone lays out the same split between inch-based and metric farm-tire markings, including width, aspect ratio, construction, and rim diameter Firestone’s tyre marking breakdown.

Width Changes Footprint And Clearance

A wider tire can spread the load over more ground and may help floatation in softer soil. It also needs room. Check fender clearance, row spacing, and the distance to steering arms or loader hardware before you swap to a wider section width.

Aspect Ratio Changes Outside Height

A 420/85R34 and a 420/70R34 share width and rim diameter, yet they do not stand the same height. The 85-series tire has a taller sidewall, so the mounted tire stands taller and rolls farther per turn.

That becomes a bigger deal on mechanical front-wheel-drive tractors. Front and rear tire sizes are paired so the rolling relationship stays in a narrow range. Change one size carelessly and driveline stress can creep in.

Construction Changes Ride And Use

Radials and bias tires may share a size family, though they do not behave the same in the field or on the road. Radials are common on larger tractors because they flex differently and are widely used in modern farm work. Bias tires still show up on older tractors, utility machines, and many implement positions.

If you are replacing one tire on a pair, match construction as well as size unless the maker of the tractor and tire allows a mixed setup for that axle.

What The Size Does Not Tell You

The sidewall size code is only the start. Load index, speed symbol, inflation pressure, and ply or star rating tell you how much weight the tire can carry at a stated speed and pressure. Bridgestone notes that the load index is the number and the speed index is the letter tied to the tire’s rated working limits Bridgestone’s load index table.

Take a tire marked with a load index and speed symbol near the size line. That marking is not decoration. It tells you the working band the tire was built for. Inflate far below the needed level for the load, and the tire can run hot, wear badly, or fail early. Inflate far above what the job calls for, and ride, traction, and soil contact can suffer.

Older bias tires may also use ply rating, such as 6PR or 8PR. That is not the same thing as the count of physical plies in every modern tire. Treat it as a strength class, then match it to the job the tractor or implement is doing.

Marking Beyond Size	What It Tells You	Why It Matters
Load Index	Rated carrying capacity	Keeps axle loads within the tire’s limit
Speed Symbol	Rated travel speed	Helps match road and field use
Ply Rating / Star Rating	Strength class on many older or heavy-duty tires	Helps match the tire to the job
Tube Type / Tubeless	Whether the tire needs a tube	Changes mounting parts and service plan
Max Load / Max Pressure	Upper printed limits	Stops guesswork during setup

How To Check A Tractor Tire Before You Order

Before you buy, copy the full sidewall code from the tire already on the tractor. Do not trust memory, and do not stop at the width and rim size. One missing slash number or one missed letter can send you to the wrong shelf.

1. Write down the full size exactly as printed.
2. Note radial or bias construction.
3. Record load index, speed symbol, and ply or star rating if shown.
4. Check the tractor manual or rim stamp for approved rim width.
5. On MFWD tractors, match the approved front and rear pairing.
6. Measure clearance if you plan to change width or profile.

If the old tire uses an inch size and the new catalog shows metric sizes, do not guess at a swap by width alone. Matching outside diameter, approved rim width, and axle pairing matters just as much as section width.

Once you read a few sidewalls this way, the code stops looking like random print. You’ll know which number tells you width, which one tells you profile, what the construction mark means, and where the rim size sits every time.