How to Read Agricultural Tire Sizes: A Complete Decoding Guide

If you've ever looked at the sidewall of a tractor tire and seen something like "520/85R42" or "18.4-38" and wondered exactly what those numbers mean, you're not alone. Agricultural tire sizing uses multiple designation systems — some metric, some imperial, some industry-specific — that can be genuinely confusing even for experienced farmers.

This guide decodes every component of an agricultural tire size designation, covers all three major systems you'll encounter, and explains how to use that information to find the right replacement tire.

Why There Are Multiple Sizing Systems

Agricultural tire sizes aren't standardized in a single format because the industry evolved across different eras and geographies:

• Older U.S. tires use a traditional inch-based system (e.g., 18.4-38 or 18.4R38)
• Modern tires — particularly European-origin brands — use the metric system (e.g., 520/85R42)
• Flotation and specialty tires sometimes use their own hybrid designations

The good news: once you understand the logic of each system, translating between them becomes straightforward.

System 1: The Traditional Inch System

You'll see this on older tires and many replacement tires for classic equipment. Example: 18.4-38 or 18.4R38

Breaking It Down:

18.4 — Section width in inches. This is the widest measurement of the tire's cross-section when mounted on the correct rim and properly inflated. In this case, 18.4 inches wide.

- or R — Construction type.

• A hyphen (-) indicates bias-ply construction
• The letter R indicates radial construction
• So 18.4-38 is bias, 18.4R38 is radial

38 — Rim diameter in inches. This is the diameter of the wheel/rim the tire is designed to mount on.

What the Inch System Doesn't Tell You:

The traditional inch system doesn't include an aspect ratio (the relationship between sidewall height and section width). This was standardized for agricultural tires of the era, so it was assumed rather than stated.

System 2: The Metric System (Most Modern Tires)

This is the standard for virtually all modern agricultural tires, especially from European manufacturers. Example: 520/85R42

Breaking It Down:

520 — Section width in millimeters. The nominal width of the tire's cross-section. (To convert to inches: 520 ÷ 25.4 = 20.5 inches)

/ — Separator between width and aspect ratio

85 — Aspect ratio as a percentage. The sidewall height is 85% of the section width. So: 520 mm × 0.85 = 442 mm sidewall height (about 17.4 inches).

R — Radial construction. Modern metric tires are almost universally radial; you'll rarely see a metric designation with a bias tire.

42 — Rim diameter in inches. (Yes, metric tires still use inch rim diameters — a legacy of agricultural equipment standardization.)

Full Calculation Example:

520/85R42:

• Section width: 520 mm = 20.5 inches
• Aspect ratio: 85% → Sidewall height = 520 × 0.85 = 442 mm = 17.4 inches
• Overall diameter = Rim diameter + (2 × sidewall height) = 42 inches + (2 × 17.4 inches) = 76.8 inches overall diameter

System 3: IF and VF Designations (Advanced Flexion)

Improved Flexion (IF) and Very High Flexion (VF) tires carry a prefix before the standard metric designation. Example: IF 650/85R38 or VF 710/70R42

The prefix tells you the tire is engineered for enhanced sidewall flexion, allowing operation at lower inflation pressures than a standard equivalent tire:

• IF = can carry equivalent load at 20% lower inflation pressure than standard
• VF = can carry equivalent load at 40% lower inflation pressure than standard

Everything else in the size designation reads identically to the standard metric system. The prefix is your signal to consult a specialized IF/VF load-inflation table rather than a standard one.

System 4: Flotation Tire Designations

Flotation tires for sprayers and grain carts sometimes use a slightly different format. Example: 380/90R50 or 710/70R42

This is still the metric system — the numbers decode identically to System 2. What makes flotation tires distinct is the wide section width (380 mm to 900+ mm) and the specific application context, not the designation format itself.

Some older flotation designations use a format like 16.9R30 (inch system) — read these using System 1.

The Load Index and Speed Symbol

After the size designation, you'll often see additional numbers and letters. Example: 520/85R42 157A8

157 — Load Index. A standardized code representing the maximum load the tire can carry at its rated speed. Load Index 157 corresponds to 7,500 kg (16,535 lbs) per tire at maximum pressure. A complete load index chart is available from the Tire and Rim Association.

A8 — Speed Symbol. The letter indicates the maximum speed the tire can safely sustain:

• A6 = max 30 km/h (18.6 mph) — standard field speed
• A8 = max 40 km/h (25 mph) — higher-speed transport
• B = max 50 km/h (31 mph) — high-speed transport tires

Some tires show two ratings separated by a slash: 157A8/154B means 157 load index at A8 speed, or 154 load index at B speed. The tire performs differently at different speeds.

The R-Code (Tread Code) — Where It Appears

As covered in our separate R1/R1W/R2 guide, the tread code is a separate designation that typically appears:

• In the tire's product name (e.g., "TM800 R1W")
• Embossed on the sidewall alongside or near the size marking
• In the specification sheet

The R-code does not appear within the size designation itself — it's a separate classification.

Cross-Reference: Inch to Metric Common Sizes

These are the most common equivalent pairs you'll encounter when shopping replacements:

Note: Metric equivalents may not be 100% interchangeable — always verify rim compatibility and overall diameter before mounting.

How to Find the Right Replacement Tire

When shopping for a replacement:

1. Read the full size off your existing tire sidewall — note every number, letter, and slash
2. Note whether it's a radial (R) or bias (-) — this affects construction type of your replacement
3. Note any IF or VF prefix — if your machine came with IF/VF tires, replacing with standard tires changes the load-inflation parameters
4. Check the load index and speed symbol — your replacement should meet or exceed both
5. Confirm rim diameter — the final number (38, 42, etc.) must match your wheel exactly

If you're unsure, our team at Farm Tires Only can cross-reference any tire size and help you identify the correct replacement from our inventory of Ascenso, Alliance Tire, Goodyear AG, Michelin AG, Mitas, Titan, and Trelleborg agricultural tires.

Farm Tires Only — store.farmtiresonly.com