Tractor Tire Size Calculator
Compare metric radial tractor tire sizes — instant diameter difference and ground-speed calibration impact, plus a standard-to-metric equivalency table.
Quick Example
A common rear tractor tire, 380/85R38 (63.4" diameter), is the metric equivalent of the classic 14.9R38. Upsizing to a flotation tire like 480/80R38 (68.2") is a +7.6% diameter change — enough to throw off wheel-speed-based ground-speed calibration on planters and sprayers if the front axle (on MFWD tractors) isn't matched proportionally. Enter your own sizes below for the exact number.
Diameter: 63.43"
Diameter: 68.24"
Diameter Difference
+7.57%
+4.80" overall diameter change
Current Revs/Mile
318
Target Revs/Mile
296
If your planter, sprayer, or yield monitor uses wheel-speed calibration (not GPS/radar), a diameter change of more than ~2% will throw off ground-speed readings and should be recalibrated.
AI Insight
Powered by AIGet a plain-English explanation of your results — what they mean for your vehicle and driving experience.
Standard ↔ Metric Equivalents (Radial Ag Tires)
| Standard (in) | Metric | Rim |
|---|---|---|
| 14.9R28 | 380/85R28 | 28" |
| 16.9R28 | 420/85R28 | 28" |
| 14.9R30 | 380/85R30 | 30" |
| 16.9R30 | 420/85R30 | 30" |
| 18.4R38 | 480/80R38 | 38" |
| 20.8R38 | 520/85R38 | 38" |
Tractor Tire Guide
Background on ag tire construction, load ratings, and general fitment concepts.
How to Use This Calculator
- Enter your current tractor tire size in metric radial format (e.g., 380/85R38).
- Enter the target tire size you're considering.
- Review the diameter difference and revs-per-mile change for ground-speed calibration.
- Use the standard-to-metric table below the calculator if your tire is labeled in inches (e.g., 14.9R38).