Tractor Tire Guide
Convert standard tractor tire sizes to metric radials with real diameters, compare tread types, and keep front and rear tire sizes matched on an MFWD tractor.
QUICK EXAMPLE
A grower on 480/80R38 rears wants a wider footprint. Swapping to 520/70R38 keeps the overall diameter at 69.1 inches, but the section width goes from 18.9 to 20.7 inches and the circumference grows from 206 to 209 inches, about 1.5% more. That pulls a 2% front lead down to roughly 0.5%, still in range, but the tire also needs a W16 rim in place of a W15, so check the wheels before ordering.
How to Read Tractor Tire Sizes
Farm tires are sold under three notations at once: inch bias, inch radial and metric radial. If you only have a code in front of you, the universal tire converter will split it into width, aspect ratio and rim size.
Inch bias (18.4-38)
The dash means bias-ply construction. 18.4 is the section width in inches and 38 is the rim diameter in inches.
Inch radial (18.4R38)
The R replaces the dash and marks radial construction. The width and rim numbers read the same way as the bias size.
Metric radial (480/80R38)
480 is the width in millimeters, 80 is the aspect ratio as a percent of width, and 38 is the rim diameter in inches.
Standard to Metric Tractor Tire Size Chart
Overall diameter and circumference are measured figures from the Firestone Ag Data Book for drive wheel tires, in inches.
| Standard size | Metric radial | Overall diameter | Circumference | Approved rim |
|---|---|---|---|---|
| 12.4R24 | 320/85R24 | 45.1 in | 137 in | W10 |
| 13.6R24 | 340/85R24 | 47.0 in | 143 in | W11 |
| 14.9R24 | 380/85R24 | 49.0 in | 148 in | W12 |
| 13.6R28 | 340/85R28 | 51.2 in | 154 in | W12 |
| 14.9R28 | 380/85R28 | 53.5 in | 162 in | W13 |
| 16.9R28 | 420/85R28 | 56.1 in | 170 in | W15 |
| 14.9R30 | 380/85R30 | 55.8 in | 167 in | W13 |
| 16.9R30 | 420/85R30 | 58.1 in | 175 in | W15 |
| 14.9R34 | 380/85R34 | 59.8 in | 179 in | W13 |
| 16.9R34 | 420/85R34 | 62.6 in | 187 in | W13 |
| 16.9R38 | 420/85R38 | 66.1 in | 199 in | W15 |
| 18.4R38 | 480/80R38 | 69.1 in | 206 in | W15 |
| 20.8R38 | 520/85R38 | 72.7 in | 215 in | W16 |
| 18.4R42 | 480/80R42 | 73.2 in | 218 in | W16 |
| 20.8R42 | 520/85R42 | 76.2 in | 228 in | W18 |
Brands and tread patterns vary a little, so treat these as a starting point. To compare any two sizes, use the tractor tire size calculator, or find a single tire's height with the overall tire height calculator.
Swapping to a Wider Tire on the Same Rim Size
A lower aspect ratio with a wider section can keep the overall height close. The figures below show how close, using the 38 inch family as an example (calculated).
| Size | Section width | Overall diameter | Circumference | Change in circumference | Approved rim |
|---|---|---|---|---|---|
| 480/80R38 (18.4R38) | 18.9 in | 69.1 in | 206 in | Baseline | W15 |
| 520/70R38 | 20.7 in | 69.1 in | 209 in | +1.5% | W16 |
| 480/70R38 | 18.9 in | 66.2 in | 200 in | -2.9% | W15 |
| 520/85R38 (20.8R38) | 21.0 in | 72.7 in | 215 in | +4.4% | W16 |
Even a 1.5% circumference change moves wheel speed readings, so recalibrate any sensor that counts wheel turns. The revolutions per mile calculator and the rolling radius calculator show the effect in numbers. A wider tire usually needs a wider rim as well, and Firestone lists W16 against W15 for this swap.
Tractor Tire Tread Types (R-1 to R-4)
Tread types follow the Tire & Rim Association code. Match the lug depth to the soil you work most, since that drives traction and how long the lugs last.
Best for: Dry to moderately moist soils, loams and firm ground
The standard farm tread. Rated up to 30 mph on A8 and B speed index tires, 40 mph on D.
Best for: Wet soils, heavy clay and residue-heavy fields
More rubber in the lugs gives better wet grip and more wear before the cleats are gone. Same speed ratings as R-1.
Best for: Standing water, muck and flooded rice or cane fields
Built for paddies. Rated to 25 mph and not meant for hard surfaces.
Best for: Lawns, turf and groundskeeping work
A wide, shallow tread that floats on turf without chunking or cutting it.
Best for: Gravel, packed dirt and concrete yards
A sturdier bar lug for tractors that spend their time on loader work and hard ground.
Front and Rear Tire Matching on MFWD Tractors (Lead and Lag)
A mechanical front-wheel drive tractor is geared so the front wheels turn a little faster than the rears. Firestone puts the front-to-rear gear ratio of a typical MFWD at 1.2 to 1.5, and the tire pair has to match it. When it does, the front wheels lead the rears slightly and pull the tractor along.
Measuring lead in the field
Lead % = (front tire turns with the front wheels engaged ÷ front tire turns with them in neutral − 1) × 100
Count the turns over the same measured distance both ways. A positive result is lead and a negative one is lag. For example, 14.3 turns engaged against 13.8 in neutral gives 4% lead.
Lag (below 0%)
The front wheels turn slower than the rears. Firestone says no amount of lag is acceptable.
Target (about 2%)
The front wheels pull without scrubbing. Anywhere from 0 to 5% lead is considered acceptable.
Too much lead (above 5%)
The front tires turn too fast and scrub the ground. Fit a front tire with a smaller diameter to bring it back.
A rear tire swap changes lead too. Going from 480/80R38 (206 in) to 520/85R38 (215 in) adds 4.4% to the rear circumference, which turns a 2% lead into about 2% lag. The tire size comparison calculator gives you the percent change for any pair before you buy. Firestone also advises keeping the front wheels in neutral on the road, since lead and lag wear is worst during road travel.
Standard Radial vs IF vs VF Tires
IF (Increased Flexion) and VF (Very High Flexion) are ETRTO tire categories. They carry more load at a given pressure than a standard radial, which lets you run lower pressure for the same load.
| Tire type | Load at the same pressure | Pressure for the same load |
|---|---|---|
| Standard radial | Baseline | Baseline |
| IF (Increased Flexion) | 20% more | 20% lower |
| VF (Very High Flexion) | 40% more | 40% lower |
Tires with a higher load index carry more at the same pressure, and the tire load index calculator explains what the number means. Older bias tires use a ply rating instead, which the load range and ply rating calculator decodes. Pressure tables are often printed in kPa, so keep the PSI to kPa converter handy. Even with a flexion tire, the load, rim and speed rating still have to be checked against the maker's table.
Tractor Tire Wear Patterns and What They Mean
Spotting an odd wear pattern early is cheaper than replacing a tire in the middle of planting or harvest.
Center lugs wearing flat
Likely cause: The tire has been run harder than its load needs, so it balloons and the crown carries too much of the load.
What to do: Weigh each axle, work out the load per tire and set cold pressure from the maker’s load table. The tractor ballast calculator gives a minimum pressure for common sizes.
Sidewall cracking or lug base tearing
Likely cause: The tire has been run soft under heavy draft, so the sidewall flexes and squirms more than it should.
What to do: Check pressure against the load actually carried, including any liquid or cast iron ballast you have added since the last time you set it.
Front tires wearing fast on the road (MFWD)
Likely cause: Too much front lead, or driving on pavement with the front wheels engaged. Firestone notes that lead and lag wear gets worse during road travel.
What to do: Put the front wheels in neutral for road travel, then measure lead and compare it with the 0 to 5 percent range.
Front wheels turning slower than the rears (lag)
Likely cause: Lag, meaning the front wheels turn slower than the rears. Firestone says no amount of lag is acceptable.
What to do: Fit a front tire with a larger rolling circumference, or a rear tire with a smaller one, until the tractor shows lead.
Pressure problems often start with weight. After adding wheel weights or tire fluid, run the new axle loads through the tractor ballast calculator to see the minimum cold pressure each tire now needs.
Frequently Asked Questions
Run Your Own Tractor Numbers
Compare two tire sizes and see the diameter and ground speed change, or work out target weights, minimum tire pressure and fluid for your tractor.
Sources: Tire dimensions, circumference, rim widths, tread types and the lead and lag guidance come from the Firestone Ag Data Book. Radial productivity and fuel figures come from Kansas State University extension publication MF-588. IF and VF load figures follow the ETRTO categories as described by tire makers.
Disclaimer:Load capacities, circumferences and pressures vary between brands and tire models. Check the tire maker's published load table and your tractor's operator manual before setting pressures or changing sizes.